Part 2

On the leaves, primary infection lesions occur on the lower leaf surfaces and are characteristically olive brown, somewhat elongated in shape and vary in size from a barely discernible dot to lesions one-fourth inch or more in diameter. Frequently, adjacent lesions coalesce, forming large very dark lesions. Primary scab lesions commonly occur on or along the leaflet veins but often may be found between the veins on the underleaf surface. On the nuts, scab lesions appear as small black dots, which are elevated or sunken in older infections. Adjacent lesions on the nuts may coalesce forming large sunken black lesions, Figure 1. When infection is severe, the entire nut surface is black in appearance, development is arrested and the nuts drop prematurely.

Pecan varieties vary in their susceptibility to scab disease. Among the highly susceptible varieties are: Burkett, Delmas, Schley, Moore, Halbert and most western varieties. Moneymaker, Success and Curtis are moderately resistant. Mahan, Stuart and Desirable varieties are highly resistant to the scab fungus. However, this character of resistance varies, depending on the area of the state, local environmental conditions and the particular strain of the scab fungus present.

Scab disease development is favored by rainy periods and cloudy days when the humidity remains high and leaf surfaces are wet. Under these conditions, spores of the fungus in contact with the wet leaf surface of a pecan leaflet or nut germinate rapidly, invade the tender tissues and initiate infection within 6 hours. Lesions resulting from these infection sites, become visible to the naked eye within 7 to 14 days, depending on environmental conditions. A period of warm dry weather after infection occurs may retard lesion development.

Control.—The control of pecan scab disease depends primarily on the protection of tender leaf, nut and shoot surfaces with proper application of an effective fungicide. A protective film of fungicide chemical prevents scab fungus infections by killing the spores immediately after their germination, thereby preventing invasion of susceptible tissues. Unfortunately, once the fungus has invaded the tissues it becomes protected from chemical attack and produces spores in great abundance. Therefore, thorough coverage of leaf, nut and shoot surfaces with a fungicide chemical must be maintained to prevent secondary infections, (, , ).

Sanitation measures, such as removal of old attached shucks and leaf stems in trees and plowing or disk harrowing under fallen leaves and shucks help reduce primary infections. See spray schedule, page 4, for scab disease control.

BROWN LEAF SPOT

The brown leaf spot disease fungus Cercospora fusca (Heald and Walf) Rand affects only mature leaves and usually does not appear until the latter part of May or mid-June. Primary lesions develop on the lower leaf surfaces as small dots, which gradually enlarge and become reddish brown with a grayish cast. The shape of the lesions may be circular or irregular, especially where two or more lesions develop adjacent to one another, Figure 2. In seasons favorable for brown leaf spot development pecan trees may be completely defoliated within 3 to 4 months if the disease is not controlled. Most pecan varieties which are maintained in a vigorous state of growth are resistant to brown spot disease.

Control.—See spray schedule, page 4.

VEIN SPOT

Vein spot disease is caused by the fungus Gnomonia nerviseda. The symptoms of the disease are similar to the leaf lesion symptoms of scab disease, but vein spot disease, unlike scab disease, affects only the leaves. Lesions of vein spot disease develop on the veins or stems of leaflets and leaves, are usually less than one-fourth inch in diameter and are characteristically dark brown to black. Leaflets and leaf stems which are severely affected drop, resulting in premature defoliation.

The fungus lives in fallen leaves over the winter. The following spring when temperature and moisture conditions are favorable, spores formed in special structures called perithecia are forcibly discharged into the air and carried by wind currents to the newly formed spring foliage, initiating primary infections.

Control.—See spray schedule, page 4.

LEAF BLOTCH

Leaf blotch disease is caused by the fungus Mycosphaerella dendroides (Cke.) Demaree and Cole. The disease occurs mainly in trees of poor vigor, which may be due to neglect, infertile soil, rosette or overcrowding. Nursery trees are particularly susceptible to the disease.

The fungus overwinters in fallen leaves. In the early spring, large numbers of spores produced in the old leaves on the ground are carried by wind currents to the young leaves in the tree, where they germinate and rapidly invade the tender leaf tissue.

The disease symptoms first appear on the undersurface of mature leaves in early summer, as small olive-green velvety spots. By midsummer black pimplelike dots become especially noticeable in the leaf spots after the surface spore masses have been removed by wind and rain, giving the diseased areas of the leaves a black, shiny appearance. When the disease is severe, infected leaflets are killed, which causes defoliation of the trees in late summer or early fall and results in reduced tree vigor and increased susceptibility to disease and insect attack.

Control.—Leaf blotch disease can be controlled effectively in the early spring by disking under old fallen leaves that harbor the fungus pathogen.

In areas where a spray program for the control of scab disease is carried out, leaf blotch usually is not a damaging disease. In localities where leaf blotch disease occurs in the absence of other pecan diseases, two applications of fungicide will control the disease effectively. The first spray should be applied after pollination when the tips of the nutlets have turned brown and the second spray application should be made 3 to 4 weeks later. See spray schedule, page 4.

CROWN GALL

Crown gall disease, caused by the bacterium Agrobacterium tumefaciens (E. F. and Town.) Conn., often is damaging to pecan trees. Nursery trees as well as trees in bearing pecan orchards are susceptible to the disease.

The development of galls is confined primarily to larger roots near the base of the tree trunk, although small roots may become infected and galls develop on them. The smaller galls are under the soil surface and cannot be detected unless the soil is carefully removed from around the roots, Figure 3. Large galls, often 10 to 18 inches in diameter, develop on larger roots and may protrude well above the surface of the soil.

Galls on nursery trees develop at or below the soil surface on the taproot and larger secondary roots.

Control.—All infected nursery trees should be dug and immediately burned. Crown gall-diseased orchard trees sometimes can be saved by digging the soil from around large roots and removing the exposed galls. Where galls were removed, the damaged root surfaces should be painted with a creosote-coal tar mixture (one part creosote to three parts coal tar) to prevent spread of the disease. Cultivation of the soil around the trunk base of infected trees should be avoided to prevent root wounds and spreading of the crown gall pathogen.

DOWNY SPOT

Downy spot disease, caused by the fungus Mycosphaerella caryigena (Ell. and Ev.) Damaree and Cole, attacks all pecan varieties. Only leaves are susceptible to the disease. Primary infection of new leaves in the spring occurs from spores produced in specialized fruiting bodies in old overwintered leaves. The downy spots appear usually during the summer months on the lower surfaces of leaflets. The downy character of the lesions is due to the production by the fungus of thousands of minute spores on the surface of each spot. The spores are spread by wind and rain to adjacent leaves and to neighboring trees. After spore dissemination is complete, the lesions visible from both leaf surfaces are one-eighth to one-fourth inch in diameter and greenish yellow. Later in the season the lesions turn brown due to the death of the leaf cells in the diseased area.

Moneymaker and Stuart varieties are most susceptible to downy spot disease although all pecan varieties are moderately to slightly susceptible.

Control.—Disk under old fallen leaves in the early spring before the leafbuds begin to swell. This practice covers the leaves with soil and prevents the discharge of spores into the air, thereby controlling primary infection of new leaves. In seasons when heavy rains make early spring disking impossible, downy spot disease can be controlled by spraying the trees as indicated in the spray schedule on page 4.

BUNCH DISEASE