Alternia Leaf Spot
Alternaria leaf spot, caused by the fungi Alternaria brassicae and A. brassicocola, is the most common late-season disease of cabbage in Connecticut and affects most other cole crops as well. The disease is introduced on infected seed or by wind-blown spores from nearby crucifer weeds, old crop residues or cull piles. Heavy, late summer dews or rains, which keep the leaves wet for more than nine hours, will allow fungi to infect new foliage. Alternaria spores are easily spread by wind, splashing rain, machinery or workers when the plants are wet.
Plants grown from seed may succumb to damping-off as seedlings, before the four leaf stage. However, the first symptom of the disease usually appears as tiny yellow specks on the oldest leaves of more mature plants. The leaf spots turn brown or black as they grow from 1/4 to 1-1/2 inches in diameter. The spots develop alternating light and dark concentric rings that give them a target-like appearance. As the disease progresses, head leaves or florets may become infected. The zonate leaf spots may extend through several leaf layers of cabbage heads or brussel sprout buds. Brown or black discoloration on the top of broccoli and cauliflower florets or curds render heads unmarketable.
Downy mildew, caused by the fungus Peronospora parasitica, is noted for its destructive capability in seed beds during cool (50o to 60oF), rainy weather. The pathogen survives between seasons in crucifer weeds, seeds and crop residues. Spores are spread to new crops primarily by wind and splashing rain. Leaf and stem spots begin as purple specks, which expand into yellowish-brown irregular patches. During moist conditions, fluffy white spore structures are visible on the underside of leaf spots. Mature cole crops, especially broccoli, may also be damaged by downy mildew. Purple or black spots and blotches appear on the inner curd of cauliflower, on the stems of broccoli florets or on the head leaves of cabbage. The inner vascular tissue of floral stems or curds may also appear discolored (brown). Infection of florets by P. parasitica may predispose broccoli to head rots caused by soft rot bacteria.
Alternaria and downy mildew fungi require living plant reservoirs, such as seeds, crop debris or a susceptible weed species, to winter over. To control these diseases, growers should skip two or three years between crucifer plantings. Control of weeds in the crucifer family is critical to prevent disease transmission to cole crops. Try to choose sites without cruciferous weeds. All crucifer seeds should be treated to reduce or eliminate these pathogens. Seeds should be placed in water at 122oF for 20 (broccoli and cauliflower) or 25 (cabbage and brussels sprouts) minutes, dried and coated with a fungicide. Seed companies will usually treat seed on request. Hot water treatment reduces seed viability, but is still one of the best means available to prevent disease spread. Alternatives to hot water treatments (such as treatment with sodium hypochlorite) are offered by several seed companies.
All diseases have the potential of being transported to a farm on infected transplants. The best alternative is to grow your own transplants from clean seed in soilless mixes or in sterilized or fumigated seed beds.
Any practice which promotes the rapid drying of foliage and soils will help minimize disease incidence. Start by choosing a planting site with good air and soil drainage. When planting, orient rows in the direction of prevailing winds for better circulation of air through the foliage. Avoid over-planting or crowding plants as this increases the drying time. In order to reduce disease spread by hand or machine, avoid working in fields while the plants are wet. Incorporating crop residues and cover crops into the soil helps maintain organic matter levels, good soil structure and drainage.
Plowing immediately after harvest helps eliminate the sources of airborne Alternaria and downy mildew spores and encourages the rapid decomposition of crop residues. Fall cabbage is a source of abundant Alternaria spores. Broccoli and cauliflower plantings should be separated from cabbage to minimize exposure of curds and florets to spores.
Plant resistant or tolerant varieties to reduce disease incidence. Bislet cabbage is listed as tolerant to Alternaria while Hybrid H is tolerant to both diseases. Arcadia, Cindy, Citation, Excalibur, Green Comet, Gypsy F1,Marathon F1, Nancy, Headline, Premium and Windsor F1 are broccoli varieties resistant to downy mildew, while Futura, Emperor Esquire, Hi-Caliber, Chancellor, Eureka and Premium Crop are tolerant. Cauliflower varieties tolerant to Downy Mildew include Cloud Nine and Candid Charm.
Available fungicides and bactericides are simply not effective enough to stop any of the major cole crop diseases, especially during prolonged periods of wet weather. Mancozeb will slow the advance and spread of Alternaria leaf spot on heading cabbage (cannot be used within five days of harvest). Aliette (fosetyl-al) is registered for downy mildew control on cole crops. Bravo (chlorothalonil), Ridomil (metalaxyl)/Bravo, and mancozeb are also registered for downy mildew control on many cole crops.
Contact your local Cooperative Extension center or refer to current recommendations for chemical control measures in the latest New England Vegetable Management Guide.
By: Jude Boucher, Vegetable Crops IPM Program Coordinator, Reviewed 2012
Reprinted from: Grower vol. 93-9, p.4-5
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