Symptoms and Signs
Rust is primarily a disease of garlic, although onion, leeks, shallots, and wild species of Allium are also hosts. Symptoms begin as small (0.04 to 0.08 inch, or 1 to 2 mm) flecks, which expand into slightly larger (0.12 to 0.2 inch, or 3 to 5 mm), oval- to diamond-shaped, reddish- to dull-orange pustules that develop on leaf blades. Lesions typically form first on older leaves, and then spread to younger leaves. Reddish airborne spores (urediniospores) are produced copiously within the lesions. Later in the growing season, the lesions may appear dark because black survival spores (teliospores) develop within the pustules. Severely affected leaves can be completely covered with pustules, turn chlorotic (yellow) and then dry out and die. When infection is severe, bulb size and quality are reduced.
Comments on the Disease
Previously, rust was a sporadic disease that caused little or no economic damage. Since 1998, however, rust has caused severe damage in some garlic-growing areas in California. In addition to garlic, onion and chives can be affected severely. However, the disease only damages onions when they are planted next to a heavily infected garlic field. Leek, elephant garlic, and shallot are more resistant.
Weedy Alliums can host the fungus, serving as a source of inoculum. The fungus probably overwinters on garlic and volunteer Allium crops.
The pathogen is active during cool weather conditions, between 57º and 75ºF (14º and 24ºC). Optimal conditions for infection occur around 59ºF (15ºC) with 100% relative humidity for at least 4 hours.
The pathogen that causes rust, Puccinia allii, is considered to be a complex of several species, and might elsewhere be referred to as P. porri, P. mixta, P. blasdalei, Uromyces ambiguous and U. durus.
Management
Rotate away from Allium crops for 2 to 3 years, keep Allium fields separated to prevent movement, and destroy volunteer Allium plants during this period. There are currently no industry garlic varieties that are completely resistant, but some California Early varieties exhibit disease tolerance. Plant seeds in well drained soils.
Fungicide sprays may be warranted if more than a few pustules develop on plants. Frequent fungicide applications are required to maintain effective control.
For optimal results, use a fungicide as a preventive application. Begin applications as soon as environmental conditions become favorable for disease development (see Comments on the 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 the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | AZOXYSTROBIN | |||
| (Quadris) | 6–12 fl oz | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Begin applications before disease development and repeat every 7 to 14 days for up to three applications. Do not apply more than one application before alternating with a fungicide with a different mode-of-action group number. | ||||
| B. | MANCOZEB | |||
| (Penncozeb 75DF) | 2–3 lb | 24 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3) | ||||
| COMMENTS: Registered for use on garlic, dry bulb onion, and shallot. Applied as a protectant and in mixes, or used in rotation with systemic fungicides for disease resistance management. | ||||
| C. | TEBUCONAZOLE | |||
| (Orius 3.6 F) | 4–6 fl oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| COMMENTS: Product needs two to four hours drying time before rain or irrigation to move systemically into the plant. After this period, the fungicide will be resistant to weathering. | ||||
| ‡ | 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 action. 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, 7, 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. |