Chapter XIV: Fungous and Other Diseases of the Pecan

The fungous diseases attacking the pecan have not been thoroughly investigated. They have not, however, become so numerous or common as to cause serious damage except in a few instances. The true fungous diseases are usually propagated and disseminated by means of spores, and the most effectual method of control usually consists in spraying with Bordeaux mixture or some other fungicide. For all fungous diseases of the pecan which may be controlled by spraying no substance will give better results than Bordeaux mixture, and directions for preparing it are given at the end of this chapter. Paris green, at the rate of four ounces to each fifty gallons of liquid, may be added to the mixture for the destruction of biting insects. For effectual work in spraying large trees, a platform should be erected on the wagon-bed to make it possible to reach the tops with the spray.

PECAN LEAF BLIGHT (_Cercospora Halstedii_): This disease of
pecan leaves causes them to turn brown, wither up and drop
prematurely. At first, small brown spots are noted. These
become larger, and at length the whole leaf is destroyed. When
attacked by this disease the tree makes no progress. An
examination of the discolored areas, under a microscope, shows
the presence of tuft-like growths of spores upon short
conidiophores. As they become matured the spores are scattered
by the rain or wind and so the disease is spread. It probably
lives over from one season to another on the diseased leaves.

The most effective remedy is to spray thoroughly three times
with Bordeaux mixture. The first application should be given
just when the young leaves are expanding, followed by two
others at intervals of two or three weeks. The fallen leaves
should, if feasible, be gathered and burned.

Pecan Scab (_Fusicladium effusum_): This disease attacks the
fruit, leaves and twigs. The husks of the diseased nuts become
covered with dark spots or specks. They become hardened and
crack open in places. As a result of the attack, growth is
stopped, the fruit does not fill out and mature, but drops
prematurely or, in some cases, remains attached to the trees
long after the leaves have fallen. Round, black spots form on
the leaves when attacked by the fungous. These become dead and
brown and in most cases the whole leaf is destroyed. When
attacked, the trees are usually so badly injured that they make
little progress. Not all varieties are subject to the disease
in the same degree and some appear to be entirely exempt.

FIG. 36. Spraying Pecan Trees.]

Those varieties which are not attacked should be given
preference in propagating work. The disease may be further
controlled by spraying with Bordeaux mixture, as directed under
leaf-blight.

PECAN ROSETTE: [L]"The earliest symptoms are a peculiar
crimping of the leaves at the ends of the branches. These
leaves are smaller with crimped margin, and when held to the
light show light green or yellow streaks between the veins. The
leaf tissue in these light-colored areas is thin and
undeveloped and often breaks away leaving angular holes in the
leaves. A tree usually shows the disease over the whole top at
once, though sometimes only a single branch is affected at
first. As the disease progresses, the foliage assumes a bunched
appearance, due to the formation of tufts of leaves at the ends
of the branches. This characteristic has led us to use the term
"Rosette" as a name for the malady.

"The next stage of the disease which is observed the second
year or later, is a dying-back of the branches from the tips.
This is followed by the development of numerous small, lateral
branches from adventitious buds. These are short, producing
thick clusters of small, unhealthy leaves, sometimes reduced to
mere skeletons, so that the rosetted appearance of the tree is
intensified. This goes on from year to year. The growth of the
tree is checked and these abnormal branches are formed only to
die back each year. Trees in the earliest stages of rosette
have been observed to have light crops of nuts, but, when badly
diseased, are barren and unsightly or worse. Rosette has been
found in all ages, from nursery stock to trees forty feet high.

"The cause of the disease remains a mystery. No fungous or
other parasite can be detected in the earliest stages. The
appearance of the trees leads us to infer that the trouble is
internal, due to some derangement of the nutritive or
assimilative functions of the plant, but we are unable to
correlate this with any corresponding external conditions. That
is to say, that so many cases have been observed on fertile
soil, when cultivation, drainage and plant food had all been
provided, that it is impossible to conclude that the disease
could be due to starvation or to the lack of any single element
in the soil, nor can it be due to over-feeding, since it occurs
in light soils and in neglected orchards.

"It seems probable that it will be classed by the plant
pathologist with peach rosette, peach yellows, and related
diseases, the causes of which still remain unknown after years
of investigation. The indications are that it is contagious,
though a complete demonstration of this point remains to be
made; at any rate, it must be regarded with concern until more
knowledge is available."

The best recommendation that can be made in regard to pecans
affected by this disease is to dig them up and burn them.

BORDEAUX MIXTURE.

Copper sulphate, 5 pounds.
Lime (unslacked), 5 pounds.
Water, 50 gallons.

Dissolve the copper sulphate in two gallons of water, place it
in barrel No. 1 and add water to make twenty-five gallons.
Slack the lime, reduce it to a very thin paste, place it in
barrel No. 2 and add water to make twenty-five gallons. To mix
the solutions of lime and copper sulphate, dip a bucketful from
each barrel, and pour together into the barrel of the spray
pump. _The two mixtures should flow together as they are poured
into the barrel._ This is one of the secrets of making a
first-class mixture. The best arrangement is to have the
barrels, Nos. 1 and 2, elevated, and use a piece of rubber hose
to run the liquids into the pump barrel.

If a large amount of spraying is to be done, a somewhat
different policy should be pursued. Too much time would be
taken up in preparing the ingredients in small quantities.
Instead, large amounts of copper sulphate should be dissolved
and large quantities of lime slacked beforehand. This may be
done as follows:

In a fifty-gallon barrel place about forty gallons of water.
Put one hundred pounds of copper sulphate in a sack and suspend
it in the water. As soon as dissolved, fill up to the
fifty-gallon mark. When well stirred, each gallon will contain
two pounds of copper sulphate. Each time some of the solution
is dipped out, the height of the remaining portion should be
marked on the inside of the barrel. Before taking more of the
solution out of the barrel, any amount of water lost by
evaporation should be made good by filling up to the mark last
made.

As soon as procured the lime should be slacked, placed in a
barrel and kept covered with an inch or two of water. In this
way it can be kept indefinitely.

To prepare Bordeaux mixture from these stock solutions, dip out
two and a half gallons of the copper sulphate solution, place
it in barrel No. 1 and dilute to twenty-five gallons. From the
slacked lime take fifteen pounds, or thereabouts, to allow for
the water it contains, reduce to a thin paste, place it in
barrel No. 2 and add water to make twenty-five gallons. Pour
the contents of barrels Nos. 1 and 2 together, as already
directed.

_Tests_: If free copper be present, severe injury may be done
to the foliage or other tender parts of the plants. Sufficient
lime should be added to neutralize it.

Dip out a small quantity in a porcelain saucer or shallow bowl,
and holding it on a level with the mouth, blow the breath
gently into it. If a thin pellicle forms on the surface, more
lime must be added. Add and test until it does not form. An
excess of lime will not hurt.

Another test is to dip the blade of a clean knife into the
mixture. If a thin film of copper forms on it after holding it
there a minute or so, more lime must be added.

Use good materials and prepare the mixtures thoroughly.

In making up the various mixtures, never use iron vessels, but
use glass, wood or crockery receptacles instead.

Strain all mixtures thoroughly into the spray pump to prevent
clogging of the pump or nozzles.

Spray thoroughly and in good season. _Be in time._

Do not use mixtures which have been leftover and allowed to
stand for some time.

FOOTNOTES:

[L] Orton, W. A., proceedings second annual convention National Nut Growers' Association, 1903, p. 82. 1904.