Foliage Diseases

Downy Spot (fungus Mycosphaerella caryigena)

Only the foliage of pecan trees is susceptible to the downy spot fungus, Mycosphaerella caryigena.

Infection occurs in the spring, near budbreak, when new leaves are infected by spores produced in old, overwintered leaves. Downy spots usually appear during the late summer months on the under surface of the 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, which are visible on both surfaces of the leaf, are 1/8 to 1/4-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. Eventually, the lesions become frosty in appearance. Moneymaker and Stuart varieties are most susceptible to downy spot disease, although all pecan varieties are moderately to slightly susceptible.

Primary infections of new leaves may be controlled by disking 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. Fungicides, applied at budbreak, will also reduce primary infections.

 

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