Problems in Growing Pecans

John R. Cole.

Several fungus diseases affect pecans. One of them, scab, is considered one of the major obstacles in pecan production and causes losses of millions of dollars to the industry annually.

The scab fungus was first found on leaves of the mocker hickory nut near Cobden, Ill., in 1882. Six years later it was found on leaves of the pecan near St. Martinsville, La. At that time growing of pecans was confined mostly to native trees in Illinois, Indiana, Tennessee, Texas, Louisiana, Oklahoma, Arkansas, and Mississippi. The first orchards were planted in Louisiana and Mississippi about 1880. By 1900 several large orchards had been planted in Georgia and Florida and orchard plantings had increased in the Southwestern States. Now the industry is extensive in the Southeast as well as the Southwest.

On the leaves, shoots, and nuts of susceptible varieties, the fungus causes premature defoliation and mummified nuts. Severely infected nuts may drop prematurely or they may die and remain attached to the shoots for an indefinite period. The scab disease is perpetuated from the old lesions. Because growing tissue only is susceptible, both foliage and nuts become immune to scab at maturity.

Pecan scab has been reported it, every State where the crop is grown, but the disease causes most damage in localities where rainfall is frequent and high humidity prevails.

Most varieties are susceptible to scab, but only in sections where the fungus thrives. Infection depends on the presence of a strain or race of the fungus capable of attacking the variety grown. Susceptible varieties may appear immune for a limited time in isolated orchards or even over extensive areas and later succumb to the disease when a suitable physiological strain of the causal organism develops or is introduced and becomes established. The various strains are becoming more uniformly distributed in areas where weather conditions favor the development and spread of the fungus.

Sanitary measures and spraying are essential for effective control of scab. Old shucks and leaf stems should be knocked off before the trees begin to leaf out in the spring. The removal of low limbs that prevent plowing near trees will aid in scab control by letting in sunlight, permit better air circulation, and so allow leaves and nuts to dry quickly after heavy dews or rains. The limbs may be removed by pruning or pasturing with livestock, especially cattle.

A large number of fungicides, including various strengths of bordeaux mixture, insoluble copper compounds, iron, and wettable sulfurs have been used in experimental work for the control of pecan scab. M. B. Waite, of the Department of Agriculture, did Pioneer work on the disease in 1909. He found that of all the materials tested, post-pollination spray applications of a home-made high-lime bordeaux mixture gave the best control of scab. I used the mixture for some years with varying success. Later J. R. Large and I tested a pre-pollination spray program and changed the formula from high-lime to low-lime bordeaux. More recently we have found that ziram and zineb, when preceded by pre-pollination applications of low-lime bordeaux mixture, give good results. They control scab, improve the condition of the foliage, and often reduce infestation of black aphids.

Timing and thoroughness of the spray applications are important. Sometimes weather conditions make it impossible to spray at the proper time to prevent early infection on the young nuts, but when that does occur and the later applications are properly applied to the trees, secondary infection can be prevented and good nuts produced. The big handicap is the cost, which limits economic control to the most favorably located orchards.

THE WORST FOLIAGE DISEASES on pecans are downy spot, brown leaf spot, pecan leaf blotch, vein spot, and liver spot. They always cause some injury to pecans in many localities. Occasionally they are a limiting factor in nut production. When pecan trees are defoliated prematurely, the nuts fail to fill and mature properly. Also, new foliage often develops, uses up food material that should be stored in the trees for the following year's crop, and prevents a set of nuts that year.

Foliage diseases usually occur in trees that are crowded or are otherwise growing under unfavorable conditions. Sometimes they may cause about as much economic damage as scab because healthy leaves are essential to the production of good crops.

A good orchard-management program that keeps the trees in a vigorous condition and the proper spacing of trees so they can take advantage of sunlight will keep down losses from foliage diseases.

Sprays to control scab will also control foliage diseases. In orchards that do not need sprays for scab, one or two applications of bordeaux mixture, ziram, or zineb will usually control the leaf diseases.

COTTON ROOT ROT is caused by a soil-inhabiting fungus and is present in Texas and other States westward to the Pacific coast. It attacks many crops. It is common in cotton and alfalfa fields and has killed pecan trees in Texas and Arizona. The disease is most active during the summer, when the fungus invades the roots and eventually causes their death. Injury to the roots reduces the moisture supply and the leaves dry out. The symptoms may occur within a few days. They may be prolonged over one or more seasons; then part or all of the foliage may be chlorotic and sparse. We know of no practical control of the root rot disease. Growers are advised not to plant pecans in soil infested with the root rot fungus, especially where cotton or alfalfa have been grown.

CROWN GALL is a bacterial disease of pecan trees in both nursery and the orchard. On trees of bearing age the disease is confined mostly to the roots and base of the tree trunk, but occasionally lateral roots also are affected. Wartlike, somewhat fragile growths, a few inches to a foot or more in diameter, are its distinguishing characteristics. Sometimes the growths extend several inches above the ground line. Because of their fragility, the galls often are broken off the roots and become scattered on top of the soil when the orchard is being cultivated.

To control crown gall, it is important to avoid using infected nursery stock, which should be destroyed, preferably by burning at the time of digging. It is suggested that the galls be removed from infected orchard trees and the wounds painted with a mixture of 1 part creosote to 3 parts coal tar. Close cultivation of diseased trees should be avoided.

A DISEASE, named bunch on account of its characteristic symptoms, was first found in 1932 on trees growing in the Red River Valley near Shreveport, La. Its economic importance has not been established. Its distinguishing symptom is the broomlike formation of branches and shoots, which may appear on small or large lateral branches and on sucker growth. In advanced cases, clusters of willowy sprouts may develop directly from the trunk or large main limbs.

The disease may appear in the lower, central, or topmost parts of the tree and then spread to the adjacent branches until the entire tree becomes diseased. We have no record of trees actually dying from the disease, but in several instances the trees were so severely diseased that the growers destroyed them.

Bunch disease is quite similar in appearance to the bunch diseases of Persian, Japanese, and native species of walnut and to black locust, all of which are caused by viruses. Furthermore, it has at least one characteristic of the phony peach disease in that phony trees usually foliate several days in advance of normal trees. In this respect bunch is also similar to peach yellows.

The geographical range known in 1953 extended east into Mississippi along the Mississippi River; west to Austin, Texas; north beyond Wewoka, Okla.; and south to Alexandria, La.

Susceptible varieties include Mahan, Schley, Burkett, Mobile, Success, Centennial, Pabst, Van Deman, Russell, and Moneymaker. The Stuart apparently is highly resistant or is a symptomless carrier of the disease. Several diseased native seedling trees have been top-worked to Stuart and the scions have remained healthy for several years.