Pecan scab, caused by the fungus _Cladosporium effusum_ (Wint.) Demaree,
is the most destructive disease of pecans in Texas. The fungus invades
the young rapidly growing shoots and leaves and later the developing
nuts. Severely infected nuts on highly scab-susceptible varieties fall
or fail to develop, resulting in a total nut crop loss. Early season
defoliation often occurs in seasons of frequent rains and high humidity
which facilitate the rapid development and spread of the scab fungus.
The scab fungus overwinters in infected shoots and in old shucks and
leaves in the trees. In the spring when temperature and moisture
conditions become favorable, the fungus begins to grow in the shoot
lesions, old leaves and shucks, and within a few days produces great
numbers of spores. These spores are spread by wind and rain to newly
developed leaves where they germinate and invade the tender tissues,
initiating primary infection. The fungus produces a great abundance of
spores on the surface of these primary infection sites and spreads
throughout the tree and infects young shoots, leaves and nuts.
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.
[Illustration: Figure 1. Scab lesions on leaves and nuts of Delmas
variety. Note concave lesions and overall scabby appearance of
severely infected nuts.]
[Illustration: Figure 1. Infected nuts.]
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, ([6], [10], [11]).
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.