Symptoms and Signs
The initial symptoms on leaves are small specks of necrotic lesions that expand with time and become necrotic blotches. Similar lesions can also develop on leaf petioles, stems, spurs, shoots, and fruit. The lesions on shoots and fruit develop a whitish layer on top, most likely caused by the waxy remnants of the separated cuticle from the killed lesion tissues underneath. All leaf, shoot, and fruit lesions are covered with solitary, dark acervuli (the conidia producing fruiting structures of the pathogen).
Under favorable disease conditions (excess rain in the spring), initial infections can be found as small necrotic lesions on the basal leaflets. Large necrotic lesions form when there are multiple lesions per blade or when the infection point is on the mid rib. Infections on the mid rib result in broken blades and torn leaf tissues. Lesions can result in leaflet yellowing and defoliation during the season, especially if the lesion is located at the base of the leaflet or there are multiple lesions. Sometimes when the petiole (leaf stem) is infected, leaflets beyond the point of infection can drop.
Severe infections of fruit can lead to patches of merged lesions.
Comments on the Disease
Anthracnose occurred sporadically for several years on black walnut in Butte, Sutter, Stanislaus, and Tehama counties. The reports of anthracnose on English walnut are generally rare in California. On English walnut, anthracnose was seen only in Lake, San Benito, and Stanislaus counties, and in recent years, reported in Sutter and Tehama counties.
The fungus overwinters on the dropped leaves. In Northern California (Hollister area), mature ascospores are produced by early April. If there is rain in April, ascospores become airborne and first infect the leaflets and later the shoots and fruit. Ascospores released from dropped leaves under the walnut trees cause primary infections, while conidia produced on leaf, stem, shoot, and fruit lesions can initiate a secondary cycle of infection. This is only when rains continue in the spring and when there is rainfall in the fall. Evidence for secondary infection includes leaf lesion infection seen in early October. Both ascospores and conidia directly infect the current growth tissues; a wound is not needed for infection to occur.
Black walnut can also be infected with the pathogen causing leaf and fruit lesions. In an orchard where Paradox and Black walnut were side by side, the black walnuts were almost entirely defoliated by early October, while the Paradox trees were only about 50% defoliated. Observations of walnut cultivars planted in the Hollister area (Northern California) indicated that Serr and Payne were the most susceptible, Howard and Tulare the least susceptible, and Hartley and Chandler showed intermediate susceptibility.
Management
Removal of leaves from the ground should reduce primary inoculum (ascospores) and subsequently reduce disease. However, no specific cultural experiments have been done with the anthracnose pathogen in walnut.
Fungicide sprays in the spring control the disease efficiently. Sprays should start when the leaf size is about half its final size and continue every 2 to 3 weeks. Three sprays in the spring are sufficient to control this disease.
| Common name | Amount per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| PREPLANT | ||||
| A. | PYRACLOSTROBIN/BOSCALID | |||
| (Pristine) | 10.5–14.5 oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone Outside Inhibitors (11) and Succinate Dehydrogenase Inhibitors (7) | ||||
| B. | AZOXYSTROBIN/PROPICONAZOLE | |||
| (Quilt Xcel) | 14–21 fl oz | 12 | 60 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone Outside Inhibitors (11) and Demethylation Inhibitors (3) | ||||
| C. | FLUOPYRAM/TEBUCONAZOLE | |||
| (Luna Experience) | 8.8–17 fl oz | 12 | 35 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate Dehydrogenase Inhibitors (7) and Demethylation Inhibitors (3) | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number. |