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.