The disease walnut anthracnose is caused by the fungal pathogen Gnomonia leptostyla (imperfect stage: Marssonina juglandis), which infects mainly eastern black walnut trees (Juglans nigra L.), whereas also the varities butternut, Persian or English walnut, Hinds walnut and California walnut get infected.
Walnut trees vary in their susceptibility for the pathogen, but even low susceptible ones become infected when optimal conditions for the fungal pathogen are given. Walnut antracnose is worldwide distributed.
Picture 1: Spores of Gnomonia leptostyla
|Anthracnose spots on a black walnut leaf.||Anthracnose spots on black walnut fruits.|
Leaves, nuts, and, occasionally, shoots are likely to be attacked. Tiny dark-brown or black spots, circular to irregularly circular, appear on infected leaves. These spots become more numerous, enlarge, and often merge to form still larger dead areas. Yellowish to golden leaf tissue usually borders these spots. Infected leaves and leaflets generally fall prematurely, but some infected leaflets usually remain attached to the tree for most of the season.
Premature defoliation affects the quality and growth of nuts. Nuts from diseased trees commonly have dark, unattractive, and shriveled meat. Sunken, necrotic spots, smaller than those on the leaves, appear on husks of infected nuts. Nuts that become diseased when immature do not develop normally and many drop prematurely. Lesions also appear on current shoots, which are attacked on rare occasions, and form dead, sunken areas that are oval to irregularly circular, and light, grayish-brown with dark, reddish-brown margins.
Life cycle of the fungal pathogen
Perithecia are formed during winter time at temperatures between 7°C-10°C. To form the sexual, spore bearing perithecia two compatible strains are needed. Within each perithecium many asci are developed, each containing eight ascospores. Ascospores are discharged from the perithecium during wet spring weather and are carried by wind to leaves. For spore discharge and dissemination Rain is needed. The optimal temperature is above 10°C. Wind and rain splash carry the spores to new green leaves, where infections take place. Symptoms are found beginning of May.
Infected leaves show black small asexual fruiting bodies (acervuli). At about 18°C the production of these acervuli is highest, they produce large numbers of conidia. Conidia cause the secondary infection source (in June- autumn). Rain splash is required as well as free moisture must remain on the leaf surface more than 6 hours. Optimal temperature for infection is about 21°C. Symptoms could be found after 2-3 weeks incubation.
Under unfavourable conditions (dry conditions) spores are able to survice some weeks. The fungal pathogen overwinters in fallen leaves (as perithecia) or on walnut twigs (spores).
Fieldclimate.com shows the probability for infections and discharge of ascospores under favourable conditions in dependence of temperature, rain and leaf moisture.