Chapter II: Plum Culture (2)

Besides the plants discussed above various nondescript members of the genus Prunus are used as stocks for cultivated plums under particular circumstances or for particular purposes. Seedlings of Munsoniana plums are supposed to be preeminently adapted for low wet lands. J. W. Kerr believes that seedlings of _Prunus hortulana_ are excellent stocks for native plums as they never sucker.[152] The Sand plum (_Prunus angustifolia watsoni_) offers possibilities as a stock for dwarfing larger growing species. According to Hansen,[153] who reviews the literature and describes several experiments of his own, the western Sand cherry (_Prunus besseyi_) dwarfs varieties worked upon it and has the merits of being extremely hardy and of producing trees which bear early and abundantly. As stated in the discussion of Subcordata, stocks of this plum have been used on the Pacific Coast and discarded because it dwarfs trees and suckers badly. According to Wickson,[154] the apricot and almond are sometimes used as stocks for plums in California and in some instances with considerable success.

Lastly, suckers are not uncommonly used by plum-growers for certain varieties. Thus in the western part of New York, the plum-growing region of the East, several varieties as the Reine Claudes and some of the Damsons are propagated from sprouts taken from the base of old trees. This method can be used, of course, only when the trees are grown upon their own roots. The writer was told by plum-growers in Germany and France that most of the plums in gardens and small plantations, constituting the majority of the plums in the two countries, were propagated from suckers. This method has small merit except that it enables a grower to get a few trees cheaply and perhaps gives a better tree of some varieties for a heavy soil. Beyond question it gives trees with a tendency to sucker—an undesirable attribute.

In the horticultural literature of the time recommendations for top-working plums are rather frequent. It is true that many varieties of plums grow slowly and make crooked growths, especially in the nursery, but in the attempts at grafting in New York the failures are more conspicuous than the successes. If top-working is decided upon, the earlier in the life of the tree it is done, the better. For the Domesticas at least, the Lombard is probably the best stock. The method of top-working is to graft in early spring or bud in late summer. Grafting ought to be used more often than it is to renew the tops of injured trees, as the difficulties in doing this are not much greater than in the case of apples.

PLUM ORCHARDS AND THEIR CARE.

Plum trees in New York are set from twelve to twenty feet apart. The amount of room given seems to depend mostly upon the custom in the locality, though, as all agree, it should depend upon the soil and the variety. The deduction which plum-growers are drawing from these experiences is that the plum should have more room than is generally given it, therefore, wider plantings are more the rule now than formerly. Little attention has been paid to mixed planting for cross-pollination in this State, as the Domesticas are planted almost exclusively and seem under orchard conditions to be self-fertile.

In this region plum trees are usually planted two years from the bud, the exception being the Japanese which are sometimes set at a year from the bud. Plum trees in the past have been headed at three or four feet above the ground but the tendency now is to head them lower—half the above distances, and in orchards so planted there seems to be no inconvenience in tilling with modern implements. In the commercial orchards of the State the heads are formed of four or five main branches and in the case of the Domesticas and Insititias about a central trunk but with the Trifloras the leader is often removed leaving a vase-formed head. After the head is formed the subsequent pruning is simple, consisting of cutting out injured and crossed branches and heading-in long, whip-like growths. The Trifloras receive more pruning than the European varieties, as much of the fruit is borne on the growth of the previous season and it is necessary to keep the bearing wood near the trunk. It is the custom to cut rank growing Trifloras severely but the value of such a procedure is doubtful, as the more such a plum is pruned the more it will need pruning in the years to follow. A better plan seems to be to curtail the food and prune as little as possible, though on rich soils the tree would probably grow out of all bounds unless cut back somewhat year after year.

About the only cultivated native plums to be found in New York, if a few Wild Goose trees here and there are excepted, are on the grounds of this Station. Experience here demonstrates that, prune as you will, certain varieties of the native species will remain crooked, ungainly and unkempt. Pruning some varieties is necessary in order to permit pickers to get into the dense, thorny heads; heading-in such varieties would make their tops wholly impenetrable.

In common with all tree-fruits the best plum orchards are tilled. Such tillage usually consists of plowing in the spring followed by frequent cultivation until the middle of August, at which time a cover-crop of clover, oats or barley is sown. The plum seems to require more water than other tree-fruits—it often thrives in comparatively moist land and fails on sandy soils where the peach would grow luxuriantly. Cultivation to save moisture is very necessary for the plum in the experience of New York growers. Grass and grain have proved ruinous in most orchards where tried, though cultivated crops between young trees to pay for keep until fruiting-time are very generally planted. The claim is made by some, and with a show of reason, that there is less of the brown-rot in tilled orchards than in neglected ones for the reason that the mummied fruits which carry the fungus through the winter are buried by plowing and with shallow cultivation, at least, do not come to light and life.

Plum-growers very generally recognize the several distinct and valuable purposes which cover-crops serve in orchards. They protect the tree from root-killing, from cold, keep the soil from washing, add humus and, with legumes, nitrogen to the soil, modify the physical structure of the soil and hasten seasonal maturity of the tree. There is one other function which is not so often taken into account. Plum orchards in which cover-crops are regularly grown, even though the crop be not a legume, need less fertilizers than those in which no such crop is grown. There are several reasonable suppositions as to why there should be such an effect, but one not usually given sufficient consideration is that cover-crops make available much plant food in the soil. Each plant in the crop collects food from soil and air, most of it otherwise unavailable, and turns it over to the trees.

A discussion of fertilizers naturally follows. Present practices in the use of fertilizers with the plum, as with other fruits, are very diverse. It is impossible to ascertain what considerations have governed the applications of fertilizers in the plum orchards of New York or what the results have been. Too often, it is to be feared, fertilizers have been used as “cure-alls” for any or all of the ills to which trees are heirs. Out of the mass of conflicting data as to the effects of fertilizers on plums, the most apparent fact is that much of the fertilizers for this fruit is wasted; this in face of the fact that plums want rich soils. But the plum crop is mostly water, the foliage remains on the ground, the trees grow several years before fruiting, their growing season is from early spring until late fall, the roots go deep and spread far, the trees transpire large amounts of water, hence may thrive on diluted solutions of plant food, and now and then there is an off year in bearing for the trees to recuperate.

It does not follow from the above consideration that plums never need fertilizers, but it does seem plain that they need rather less than truck or farm crops and that applications of plant food must be made with exceedingly great care if fertilizing is to be done without waste. There is a growing disposition on the part of plum-growers to experiment very carefully and know that they are getting the worth of their money before using any considerable quantity of fertilizers for their trees.

Thinning the fruit should be a regular practice with plum-growers, but it is the operation in the growing of this fruit about which growers are most careless both as to whether it is done at all and in the manner of doing. Many growers in New York, realizing the great necessity of thinning certain varieties of Triflora, as Burbank and Abundance, follow the practice very regularly with plums of this group; but the Domesticas are seldom well thinned, though some of them, of which Lombard is a conspicuous example, ought nearly always to have anywhere from one-fifth to half of the fruit removed. Growers of some of the native varieties in regions where these sorts are grown say that under cultivation some kinds of these plums will bear themselves to death if a part of the crop be not removed in most years. Those growers in New York who thin, do the work as soon as possible after the June drop has taken place.

HARVESTING AND MARKETING.

Plum trees in this climate begin to bear when set from three to five years. The Triflora varieties will bear soonest, the Old World varieties next in order, say at four years from setting, and the native sorts, as a rule, come in bearing last. At eight or ten years of age, prolific varieties of the Triflora and Domestica sorts bear in a good year about three bushels of fruit; the Insititia and native varieties, on the Station grounds, at least, do not bear as much, though most of the plums of these two groups bear more regularly than the first named groups.

Plums in this State, and east of the Mississippi generally, are picked and put upon the market just before they reach edible condition; while farther away they must be picked much greener. It is the practice in the East to pick while still somewhat green because the fruit so picked is best handled at this stage of maturity and the brown-rot fungus is likely to destroy much of the crop if left until fully matured. Some of the Triflora sorts, Abundance, Burbank and October, for example, are picked from a week to ten days before ripe and yet develop very good color and flavor. The Domesticas need not be, and are not, picked quite so green. In picking, great diversity exists as to ladders, receptacles, and manner of conveyance from orchard to packing house. These need not be discussed here, nor need the methods of picking be spoken of further than to say that while good growers consider it vital not to bruise the fruit nor destroy the delicate bloom, if such injuries can be avoided, pickers in general are not nearly as observant of these important details as they should be.

The plum crop is sent to market, for most part in New York, in six, eight and ten-pound grape baskets with the preference at present for the smallest of these baskets. Occasionally some fruit is packed in four pound baskets. Rarely, and always to the disadvantage of both producer and consumer, plums go to market in the packages in which the fruit is picked. Indeed, it is seldom advantageous to pack the fruit in the field, it being far better to convey it to the packing house where the preparations for shipping may be more carefully made, as the package and the manner of packing advertise the product. Plums coming to this State from the far West are often wrapped individually in tissue paper as a help in safe shipping and to add to their attractiveness but the fruit grown in the State is seldom, if ever, so treated, though it is possible that choice specimens could be profitably wrapped. Of the sorting, grading, facing and marking the packages, little need be said except that they are too rarely well done in present methods, though there is a steady improvement in attending to these important matters.

Few plums are stored longer than a week at most in common storage and three weeks or a month is quite the limit for most varieties in cold storage. Late plums and in particular some of the prunes might well be stored longer than is now the custom if proper precautions are taken, as is shown by the experience at this Station where a considerable number of the Domestica and Insititia varieties are annually kept in common storage for a month or longer without unusual precautions. Some of the new varieties offered to growers, as Apple and Occident, are recommended as keeping for several weeks after picking. There is a most marked difference in the keeping qualities of this fruit and it is certain that varieties can now be selected for long keeping and that there is a fine opportunity for breeding sorts that will keep even longer than any we now have.

Marketing, the actual selling, is a business quite by itself, and since it is one which has changed greatly in the past few years and is destined to change even more in the near future, a few observations on the subject are worth putting on record. A well developed local market is undoubtedly the best selling place for the plum producer, as in it the sales are directly to the consumer, eliminating expensive middlemen. The westward spread of manufacturing industries, the workers in which use up the western-grown fruit, is making better local markets for eastern plums, a point worth noting, for many New York plum-growers have ceased planting, indeed have been removing trees, fearing western competition.

By far the greater part of the plum crop now finds its way to consumers through the following costly distributive system: 1st. Local buyers who ship to centers of consumption. 2nd. Transportation companies. 3rd. Commission companies who collect and distribute the crop in consuming centers. 4th. Retailers who parcel out the quantities and the qualities demanded by the consumer. The great defect in handling the crop is, that there are too many men and too much machinery to do the work cheaply—moreover, the risks of depreciation are great, and the fruit is not handled on a large scale chiefly because of a lack of capital by the grower or local buyer. These defects in the present distribution of plums in New York make the price received by the grower about half that paid by the consumer and the selling of the crop a more or less speculative business. The plum industry, as is the case with all fruits, is greatly hampered by the present marketing systems.

Unfortunately there is yet but a small outlet for surplus plums as manufactured products. As a rule the commercial outlook is best for those fruits of which the surplus can be turned into by-products. The only outlet for the plum in the East is in canning, as this region is unable to compete with the West in the making of prunes[155] and as the several plum products of the Old World are not in demand in the New World. Beyond question there are a number of products, as preserves, jellies from the native plums, glacé fruits, plum butter, marmalades and the like, which could be made profitable for the markets and thus a great help in utilizing surplus plums.

DISEASES.

Plums are subject to a considerable number of fungus diseases, several of which are often virulent, the virulence depending on locality, season, weather and variety. Happily for the plum industry, knowledge of plant pathology has made such advancements in recent years that nearly all of the diseases of this fruit are now controlled by preventive or remedial measures.

One of the commonest and most striking of the diseases of the plum is black-knot[156] (_Plowrightia morbosa_ (Schw.) Saccardo) characterized by wartlike excrescences on shoots and branches. In early summer these knots are dark green, soft and velvety, but as the fungus ripens in the fall the color changes to a carbon-like black and the knots become hard and brittle. The disease is usually confined to one side of the twig or branch so that death of the affected part does not ensue at once. Black-knot is an American malady, at one time confined to the eastern part of the continent where in some localities its ravages forced the abandonment of plum-growing. The fungus is now endemic to wild or cultivated plants in practically all the plum-growing regions of the continent, but it is still epidemic only in the East, the South and West being practically free from the disease. Unless especially virulent black-knot is controlled by cutting out the diseased wood. Usually eradication is not possible without several prunings during a season.

Much has been made of the supposed immunity of some varieties of plums to black-knot. In the vicinity of this Station, where the disease is always present and often rampant, the differences in immunity are not very marked in varieties of the same species. The Trifloras are less attacked, however, than any other group of plums, and the Insititias rank next in immunity. No variety of the Domesticas has yet proved to be free from the disease but strong claims are made that Middleburg and Palatine are relatively free.

Next in order of seriousness among the diseases which attack cultivated plums is the brown-rot[157] (_Sclerotinia fructigena_ (Persoon) Schroeter) known also very commonly as the ripe-rot and sometimes as peach-blight. The disease is most conspicuous on the ripe fruits of the various drupes and is popularly supposed to be confined to the fruits alone. Such is not the case, for it also attacks, and very vigorously oftentimes, the flowers and shoots. The presence of the disease on the fruits is known by a dark discoloration of the skin which is afterward partly or wholly covered by pustule-like aggregations of grayish spores. The decayed fruits may fall to the ground, or as is more usual in the case of plums, they hang to the tree and as the juice evaporates become shriveled mummies, each mummy being a storehouse of the fungus from which infection spreads the following season. The twigs, flowers and leaves are known to be suffering from inroads of the parasite when they are blackened as if nipped by frost. In warm, damp weather the rot spreads with great rapidity and fruits touching in clusters or in boxes stored for shipping are well placed to spread the epidemic. Destruction of the mummy-like fruits and all other sources of infection, and spraying with bordeaux mixture are now practiced as preventives, but so far as the crop is concerned with but indifferent success. A better remedy than we now have is eagerly looked for by growers of fruits.

The hosts of this fungus show varying degrees of susceptibility to it, the peach and the sweet cherries being more subject to it than plums. Similarly, among plums some species and varieties are more susceptible than others. Thus the Trifloras and Americanas, the latter especially in the South, are injured more by the brown-rot than other species. The idiosyncrasies of varieties in this regard are best shown in the discussions of the individual sorts.

Several interesting and sometimes destructive diseases of plums are caused by various species of the fungal genus Exoascus.[158] The most common of these, and the most striking and destructive, is plum-pockets (_Exoascus pruni_ Fuckel), which causes prominent deformities of the fruit. These give the disease the common name or less frequently “bladders” and “curl.” The fungus attacks the developing fruits at an early stage of their growth and causes the production of a spongy mass in the fleshy tissue which greatly enlarges and distorts the plum. The stone in a diseased plum is but rudimentary or very often not at all developed. Less prominently but quite as frequently, the leaves are attacked, showing as they unfold more or less red or yellow with a very decided curling and arching of the leaf-blade. The disease usually spreads from the leaves to the shoots, the infected shoots with their rosettes of mal-formed leaves giving the tree a most unsightly appearance. Prevention at present consists of removing the diseased parts and spraying with bordeaux mixture when the buds begin to swell. Munsoniana and Hortulana plums seem to be most susceptible to this disease. Atkinson[159] has described several species of Exoascus on the different species of wild plums, some of which are liable to be found on the cultivated varieties of the native plums. They are all very similar to _Exoascus pruni_, differing chiefly, in the eyes of the layman, in forming smaller pockets. Sturgis[160] records an attack of one of the leaf-curl fungi, distinct from the plum-pockets fungus, on varieties of Triflora in Connecticut, which seemed to him to be of scientific and economic importance.

The leaves of the different species of cultivated plums are attacked by several fungi which produce diseased spots on the foliage, which for most part drop out, causing a shot-hole effect. These diseases pass under such descriptive names as “shot-hole fungus,” “leaf-spot,” and “leaf-blight.” The fungus probably responsible for most of this trouble is best known as the shot-hole fungus[161] (_Cylindrosporium padi_ Karsten). The Domestica and Triflora varieties are very susceptible to this fungus, which, on the foliage of the first, causes spots for most part, while on the latter the spots on the leaves are nearly always followed by holes. Varieties of the native species, especially those of Americana and Nigra, are relatively free from this disease. Another of these shot-hole fungi is _Cercospora circumscissa_ Saccardo[162], much less common than the former, but still to be considered and especially on the foliage of Americana. All of these diseases of the foliage are prevented to some degree by the proper use of bordeaux mixture, which, on the Triflora plums at least, must be used with great care to avoid injury. Cultivation has a salutary effect as it destroys the diseased leaves which harbor the fungi.

Another disease of plum foliage, occurring rarely on the fruit, is the plum-leaf rust[163] (_Puccinia pruni-spinosae_ Persoon) which produces so considerable a number of spore cases on the underside of the leaves as to give the foliage a brownish cast and to cause defoliation in severe infections. The fungus is most apparent in the fall and most troublesome in warm, moist climates. Bordeaux is used as a preventive.

Stewart and Rolfs have shown that trunks and branches of plums affected by sunscald in New York are almost invariably infested by a fungus[164] (_Valsa leucostoma_ Persoon) which in the Old World is known as the “die back” of the peach. The disease manifests itself on plums chiefly by affected areas much depressed at the boundary between the living and the dead bark, these areas usually, not always, having connection with sunscald injuries on the trunk. The disease is accompanied by more or less gumming.

In common with nearly all rosaceous plants, in nearly all countries, the plum is sometimes seriously injured by the powdery mildew[165] (_Podosphaera oxyacanthae_ DeBary). The affected leaves have a grayish appearance caused by the parts of the fungus which project beyond the leaf tissue; when badly diseased the leaves are more or less arched and curled. Mildew is seldom prevalent enough on plums to require treatment.

The crown gall,[166] (_Bacterium tumefaciens_ Smith and Townsend) is a parasite on all of the fruits of the order Rosaceae and is especially common on nursery stock, attacking plums in many soils but rarely, however, to the great injury of the plant. These galls are perennial structures of very varying duration. They are to be found on the roots, usually at the collar of the plant, and vary from the size of a pea to that of a man’s fist, forming at maturity, rough, knotty, dark-colored masses. Means of prevention or cure are not established though all agree that soils may be inoculated with the disease from infected stock; hence the necessity of discarding diseased trees at transplanting time.

Smith found in Michigan and Clinton in Connecticut a disease of the fruit called bacterial black spot[167] (_Pseudomonas pruni_ Smith) of the same generic origin as the crown gall but widely different in nature. The writers and the growers who found the infected fruit, saw the disease only on the Triflora plums. It attacks the green fruits which show conspicuous, black-purple, sunken spots sometimes as large as half an inch in diameter. The injuries are usually isolated and quite superficial but nevertheless, spoil the fruit.

The plum in common with other stone-fruits often suffers from an excessive flow of gum, for which trouble the name gummosis[168] is now generally applied. The disease is to be found wherever plums are grown but it is much more destructive on the Pacific than on the Atlantic seaboard. So far as is now known gummosis is secondary to injuries caused by fungi, bacteria, insects, frost, sunscald, and mechanical agencies. The disease is least common in species and varieties having hard wood; on trees on soils favoring the maturity of wood; under conditions where sun and frost are not injurious; and, obviously, in orchards where by good care the primary causes of gumming are kept out. Stewart[169] has recorded an interesting case of gum-pockets in the fruit, but could assign no cause.

Mechanical injuries from the sun, frost and hail are troubles with which nearly all plum-growers must contend at one time or another. In this region the Reine Claude and Triflora plums suffer much from sunscald but none are wholly immune, though Lombard is possibly most so. These injuries from the elements of weather are often mistaken for diseases, and are so often followed by fungal parasites and insects as to make it difficult to distinguish the primary from the secondary trouble. Low-heading of the trees is the best preventive of these trunk injuries.

Plums are somewhat subject to attacks of the well-known peach scab[170] (_Cladosporium carpophilum_ Thumen). The scab appears in numerous, small, sooty, circular spots of brownish color, often confined to one side of a fruit but in other cases distributed over the whole surface. None of the cultivated species are free from the disease but the Munsoniana, and Hortulana varieties are most susceptible to it. Pear blight,[171] (_Bacillus amylovorus_ (Burrill) DeToni) commonly thought of as a disease of the pear and apple has been found on various plums, and the yellows of the peach, cause unknown, is often quite destructive to Triflora plums. According to Smith the peach rosette,[172] cause unknown, attacks both wild and cultivated plums in the South and is quickly fatal. The disease was prevalent on the wild Angustifolias, on two varieties of Triflora, Kelsey and Botan, but the observer had not seen rosette on varieties of Domestica.

Waugh describes a trouble which he calls “flyspeck fungus”[173] found on fruits sent from the Southern States, in which small areas are thickly dotted with black spots; also a fruit-spot on plums from Texas caused, as he states, by an undetermined Phoma.[174] Starnes of Georgia describes a malady of the Triflora plums called “wilt,”[175] cause unknown, which he states is the most serious obstacle to the culture of this plum in the South. In this peculiar disease the foliage passes directly from a green, healthy state into a wilted and then parched condition, the death warrant being signed when a tree is once affected. In Oregon and Washington the Italian Prune is subject to a leaf-curl[176] which begins in mid-summer and curls the leaves conduplicately without withering but shriveling somewhat. As the season advances the leaves turn yellow and many of them drop. Neither cause nor cure is known. Smith described a plum-blight[177] of native plums in Georgia which “destroys large branches or even whole trees in mid-summer in the course of a few weeks.”

INSECTS.

Cultivated plums furnish food for a great number of insects. Many of the destructive insect pests of the several cultivated species of Prunus are known to have come from the wild plants of the genus, but others, and possibly the majority, come from over the seas. No less than forty species of insects may be enumerated as pests of the plum and many more can be counted as occasional parasites on one or another of the species. Of the formidable pests the plum curculio is probably the most troublesome. The plum curculio[178] (_Conotrachelus nenuphar_ Herbst) is a rough, grayish snout-beetle somewhat less than a quarter of an inch in length, an insect so familiar to fruit-growers as hardly to need a description. The female beetle pierces the skin of the young plums and places an egg in the puncture. About this cavity she gouges out a crescent-shaped trench, the puncture and trench making the “star and crescent” of the Ottoman Empire, hence the common name of the beetle, “The Little Turk.” The egg-laying process may be repeated in a number of fruits and from each egg a larva hatches within a week and burrows to the stone, making a wormy fruit. Most of the infested plums drop. In years past plum-growers relied upon jarring the beetles from the trees in the early morning, but the treatment was too expensive, and poisoning with an arsenate is now the chief means of combating the pest. Rubbish and vegetation offer hiding places for the insects and hence cultivated orchards are more free from curculio. Thin skinned varieties are damaged most by the insect but there are no “curculio-proof” plums.

A larger snout-beetle than the curculio, the plum gouger[179] (_Anthonomus scutellaris_ LeConte), occasionally does much damage to plums. The work of the gouger may be told from that of the curculio by the absence of the crescent cut about the puncture made for the egg, and from the fact that the larvæ of this pest chiefly infest the stone and those of the other insect the flesh of the plum. The remedies are the same for the two insects though the gouger is more easily destroyed.

Among the several borers which are more or less destructive to species of Rosaceae only the peach borer[180] (_Sanninoidea exitiosa_ Say.) may be counted as a troublesome pest of the plum. The larvæ of this insect are frequently to be found in both wild and cultivated plum trees and must be combated in nearly all plum orchards east of the Rocky Mountains. The prevention of the work of the borer is best accomplished by thorough cultivation, the use of coverings of tar and poisonous washes and mounding the trees. Destruction is effectively carried out only by digging out the borer with knife or wire. The lesser peach borer[181] (_Sesia pictipes_ Grote & Robinson) attacks the plum and in New York has been found particularly injurious to the Wickson plums. The flat-headed apple tree borer[182] (_Chrysobothris femorata_ Fabricius) is frequently found in the wood of wild plums and is sometimes a pest of the several cultivated species. It is treated as is the peach borer. The shot-hole borer[183] (_Eccoptogaster rugulosus_ Ratzeburg) a diminutive insect which deposits its eggs in the trunks or large branches of various members of the genus Prunus, may be regarded as an effect rather than a cause of disease, for it seldom injures perfectly healthy plum trees. The peach bark-beetle[184] (_Phlœotribus liminaris_ Harris) is somewhat similar in its work to the shot-hole borer and like it is found for most part only in diseased and decrepit trees.

The plum aphis[185] (_Aphis prunifolii_ Fitch) is sometimes very destructive to varieties of the native plums, especially the Americanas, and occasionally injures or even kills the young trees of the Domestica sorts. It is not a formidable foe in New York, and it is the exception when trees must be treated for it, the treatment being any of the contact solutions used against sucking insects. The cherry aphis[186] (_Myzus cerasi_ Linnaeus) and the green peach aphis[187] (_Myzus persicæ_ Sulzer) are much less common than the plum aphis on plum trees, but are sometimes abundant on foliage of this fruit and are combated in the same way as the more common aphis. Gillette enumerates two other aphids as attacking the plum in Colorado—the rusty brown plum louse[188] (_Aphis setariæ_ Thomas) and the mealy plum louse[189] (_Hyalopterus arundinis_ Fabricius).

Several scale insects infest the plum. Chief of these is the dreaded San José scale[190] (_Aspidiotus perniciosus_ Comstock) known and feared by all fruit-growers in the United States. The lime and sulphur solution is now the most common and probably the most effective spray for this insect. The European fruit lecanium[191] (_Lecanium corni_ Bouche) occasionally does a great deal of damage in New York and now and then destroys the whole crop in an orchard. The winter treatment for San José scale is used to control this pest, but usually such treatment is supplemented by a summer spray about July first with such contact sprays as whale oil soap and kerosene emulsion. Of the other scales[192] which feed upon plums and now and then become pestiferous the following may be named: The fruit pulvinaria (_Pulvinaria amygdali_ Cockerell), the mealy bug (_Pseudococcus longispinus_ Targioni), the scurfy scale (_Chionaspis furfura_ Fitch.), the West Indian peach scale (_Aulacaspis pentagona_ Targioni), the Putnam scale (_Aspidiotus ancylus_ Putnam), the cherry scale (_Aspidiotus forbesi_ Johnson), the walnut scale (_Aspidiotus juglans-regiæ_ Comstock), Howard’s scale (_Aspidiotus howardii_ Cockerell), the European fruit scale (_Aspidiotus ostreæformis_ Curtis), the red scale of California (_Chrysomphalus aurantii_ Maskell), the oyster-shell scale (_Lepidosaphes ulmi_ Linnaeus), and the soft scale (_Coccus hesperidum_ Linnaeus).

Wild plums of several species seem to be favorite feeding grounds for the tent caterpillar[193] (_Malacosoma americana_ Fabricius), but cultivated plums are not so often attacked; the spraying with arsenites usually given for the curculio is fatal to this pest as well. The spring canker-worm[194] (_Paleacrita vernata_ Peck) and the fall canker-worm[195] (_Alsophila pometaria_ Harris) are other caterpillars which often do much damage unless checked by destructive measures, of which the best are the arsenical sprays. The larvæ of a considerable number of other moths and butterflies are often found on plum foliage but seldom in such numbers as to require systematic destruction.

The eye-spotted bud-moth[196] (_Tmetocera ocellana_ Schiffermuller) is a rather serious pest in plum nurseries where the larvæ eat into the young buds at the time they are opening, often destroying the shoots. Sometimes the larvæ are found destroying buds on old trees. The arsenical sprays are used to destroy this pest. The peach twig-moth (_Anarsia lineatella_ Zeller) is now and then found in plum twigs but is seldom classed as a plum pest.

The clover mite[197] (_Bryobia pratensis_ Garman) formerly considered a serious pest of the plum only in the far West, is now abundant in plum orchards in New York. It can be controlled by applications of lime and sulphur. The bumble-flower beetle[198] (_Euphoria inda_ Linnaeus) is reported by Goff to be occasionally very injurious to plums in Wisconsin. Remedies seem not to have been tried. The larvæ of the cherry slug[199] (_Selandria cerasi_ Peck) is not infrequently found doing damage to plum foliage and occasionally the larvæ of one or two other saw-flies feed on the plum. These are all overcome by the use of arsenical sprays. At least one of the curious insects know as “case-bearers” attacks the plums. The troublesome one is the pistol case-bearer[200] (_Coleophora malivorella_ Riley), which is not often a pest but has sometimes done considerable damage. Attempts to check this insect with the arsenical sprays have usually proved successful. The plum tree is a host-plant of the hop aphis[201] (_Phorodon humuli_ Schrank). So much does this insect feed on the plum that the destruction of plum trees in the vicinity of hop fields is recommended to hop-growers by Riley.