Part 3

Although the cause of bunch disease is not known, evidence indicates it is an infectious disease, which suggests that the causal agent may be a virus.

Trees affected with bunch disease show the bunching symptom, which is due to excessive growth of slender succulent twigs from lateral buds that normally remain dormant. In moderately affected trees one or several branches will show the “bunch” growth symptom. Bunching in severely affected trees may involve all main branches which produce thick masses of sucker-like growth and few, if any, nuts.

Observations indicate that the Stuart variety is the most resistant to bunch disease.

Control.—There is no known effective control for bunch disease. Early detection of the first symptom of bunch and pruning out of the affected branch may prevent spread of the disease throughout the tree. When the tree is severely affected, and limbs are involved, the tree should be destroyed to protect nearby healthy trees from infection.

For propagation purposes, all bud or scion wood should be taken only from bunch disease-free trees.

ROSETTE

Rosette is a nutritional deficiency disease caused by certain soil conditions which make zinc unavailable to the pecan tree. All pecan trees require zinc for growth.

Trees showing the first symptom of zinc deficiency have yellowed tops. The individual leaflets when examined are yellowish and mottled. The next season the foliage may be yellowish and the leaflets narrowed and crinkled. More severely affected trees produce foliage which is a yellowish to reddish-brown overall color, and the leaflets are very narrow with reddish-brown spots and may be perforated. Shoots are much shortened and the leaves are produced in compact bunches of dense foliage.

Trees affected by rosette for several seasons have many dead shoots and small branches from the dying-back of each season’s growth, Figure 4. Such trees are greatly stunted, of poor vigor and produce few, if any, nuts.

Control.—Rosette is controlled readily by applying zinc sulfate to the tree either as a foliage spray or in the dry form as a soil application. Where a disease and insect spray control program is being carried out, zinc sulfate may be added to the spray mixture.

Foliage spray. Two pounds zinc sulfate (36 percent) per 100 gallons of water.

First application: after pollination when tips of nutlets turn brown.

Second application: 3 to 4 weeks later.

Third application: 3 to 4 weeks later.

Soil application. Application of zinc sulfate to the soil, particularly in a large orchard is a more expensive operation, but it provides longer protection against rosette.

In highly alkaline soils, or soils that readily fix zinc and make it unavailable to the tree, foliage spray applications of zinc sulfate are more economical because of the excessive rates required to supply available zinc through the soil.

Rate of application of zinc sulfate: Mildly rosetted trees—apply 5 pounds zinc sulfate (36 percent) annually for 2 to 3 years. Severely rosetted trees—apply 5 to 10 pounds zinc sulfate (36 percent) annually until rosette symptoms disappear.

Time and method of application: Apply zinc sulfate to the soil around trees in late February or early March. Broadcast zinc sulfate under the tree from the trunk to several feet beyond the limb canopy. Disking, harrowing, or any operation that mixes the zinc sulfate with the soil, is desirable to prevent washing away and surface soil fixing of zinc.

LICHENS

Lichens commonly are found growing on the branches and trunks of pecan trees, especially in humid areas and river bottom orchards having poor air drainage.

Lichens are nonparasitic to the pecan tree, but merely attach themselves to the bark surfaces. Lichens grow equally well on rocks, fence posts, bricks and other objects. There are several types of lichens that occur on pecan trees, none of which are damaging except perhaps in appearance to the trees in cases of extremely heavy infestations, Figure 5.

Control.—The occurrence of lichens in trees regularly sprayed with copper-containing fungicides is rare.

ARTICULARIA LEAF MOLD

Articularia leaf mold caused by the fungus Articularia quercina (PK) Hoehn is a disease of minor occurrence and importance. The disease occurs most commonly following rainy periods and in areas of high relative humidity in the leaves of trees of poor vigor.

The fungus produces on the lower surfaces of the leaves a conspicuous growth of white tufts which contain masses of spores, Figure 6.

Control.—Articularia leaf mold does not occur in trees or in orchards which have been sprayed for disease control.

A single application of fungicide such as zineb at 2 pounds per 100 gallons of water when the disease is first detected is usually sufficient to control Articularia leaf mold disease.

PINK MOLD