Symptoms and Signs
Rust is most common on fern growth after the harvest season is over. Infections begin in spring from spores that overwintered on crop debris. These infections produce the orange stage (pycnia and aecia) of the disease. Occasionally, this stage can be found in spring on emerging spears from new or established plantings. The orange stage is characterized by light green patches on new spears that mature into yellow or pale orange pustules in concentric ring patterns. Spores produced by these spring stages are airborne to new fern growth. Infection occurs and brick-red pustules develop on stalks, branches, and leaves of the fern. These red pustules produce airborne, rust-colored spores (urediospores) in a powdery mass, which can reinfect the fern and increase disease incidence. Fern yellowing and browning, defoliation, and dieback may occur. As ferns mature and senesce, or autumn weather begins, the black spore stage may develop. The same pustules that produced the red spores begin producing black spores (teliospores). The pustule will slowly convert in appearance to a powdery mass of jet-black spores. These black spores are the overwintering stage of the fungus.
The overall effect of rust on asparagus is reduced plant vigor the following year and reduced yields.
Comments on the Disease
Rust diseases have several stages, some of which may occur on different hosts. In asparagus rust, however, all the life stages (orange spore in spring, red spore in summer, and black spore in fall and winter) occur on asparagus. Therefore, what may appear to be a different disease, could be a different stage of rust.
Rust is favored by temperatures between 55° and 90°F. Several hours of dew or rain (free water) are necessary for spores to germinate and infect the host.
Good field sanitation and irrigation practices are important components of managing rust. Treatments are necessary when monitoring indicates rust is present.
Provide adequate irrigation during the spring and summer fern period so that plants are neither over- nor under-watered. Orient rows with the prevailing wind, if possible, to allow free flow of air through the field. This will allow faster drying of the soil surface when irrigations or rainfall occur. At the end of the fern season, cut and destroy diseased ferns. One of the best solutions is to incorporate the cut fern with a power-driven rotary tiller two times, once in each direction. The fern may also be removed from the field. Cut young spears to keep infections from occurring, thus breaking the cycle of the fungus in spring. Destroy volunteer asparagus within 400 yards of commercial asparagus fields.
Organically Acceptable Methods
Cultural control and sulfur dust treatments are acceptable to use in an organically certified crop.
Monitoring and Treatment Decisions
Monitor spears and ferns for the appearance of rust lesions. Begin treatments when rust first appears.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following materials 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.|
|(Chlorothalonil 720 SC)||2–4 pt||12||See comments|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)|
|COMMENTS: Do not apply within 120 days of spear harvest. Research from Michigan indicates very effective.|
|(Rally 40WSP)||5 oz||24||See label|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)|
|COMMENTS: Begin applications to the developing ferns after harvest has taken place. See label for restrictions.|
|(Penncozeb 75DF, Dithane DF Rainshield)||2 lb||24||120|
|(Manzate Max)||1.6 qt||24||120|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)|
|COMMENTS: Apply at first sign of rust and repeat at 10-day intervals until disease pressure subsides. Do not apply during harvest. Do not apply more than 6.4 lb a.i./season. Apply only on asparagus ferns after spears have been harvested.|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)|
|COMMENTS: Although this material is registered, it does not provide very effective control. Use after cutting stops. May repeat at 7- to 10-day intervals throughout the season.|
|**||See label for dilution rates.|
|‡||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, 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 a fungicide with a different mode-of-action group number.|
|#||Acceptable for organically grown produce.|