One Darn Good Reason to Rotate: Phytophthora

The Rutgers Extension Pathology Specialist, Dr. Stephen Johnston, estimated that more than 25% of the New Jersey pepper crop, worth more than five million dollars, was lost to Phytophthora blight alone during the extremely wet 1996 season. Here in Connecticut, several growers experienced near complete loss of pepper, tomato (solanaceous) and vine crop (cucurbits) fields by August of last year due to this disease. Some of the fields that were lost to this disease had been fumigated and were sprayed continuously throughout the season with a combination of fungicides. This disease was present to some degree at almost every Connecticut farm I visited last summer.

This blight, caused by the fungi Phytophthora capsici, or other related species, has been working its way toward the Northeast since it was first identified in New Mexico in 1918. Phytophthora blight often causes stem or branch lesions that first appear water-soaked but rapidly girdle and kill either the entire plant or the infected limb beyond the wound. The first symptom a grower may notice is the rapid wilting and death of plants following heavy rains (>2″). The disease usually starts in low areas of fields that remain flooded for at least 48 hours. Infected fruit may produce a white fungal growth that resembles shaving cream (peppers) or completely covers the surface with a thin layer of spores (pumpkins, squash, etc.). On tomatoes, the fruits develop a firm, brown ring which renders them unmarketable. Successive heavy rainfalls can spread secondary spores throughout a field or on the wind to nearby sites.

The good news is that, in most Connecticut fields last year, the damage was restricted primarily to a few plants around the low wet holes. The bad news is that if you plant a susceptible crop like cucurbits or solanaceous plants, and we have another wet year, this disease can easily spread and build up in your soil to cause extensive destruction. This disease has a true soil borne resting spore, meaning, that it can survive in the absence of the host crops for many years. Once the spore numbers rise to destructive levels throughout the field, even long rotations of five or more years will not necessarily reduce the spore count. This past season demonstrated that even heavy use of all known chemical controls will not adequately protect your crop in highly infested fields during wet seasons. That means the only sure control for Phytophthora blight is prevention: don’t let it build to high levels in your fields!

Management: Rotate OUT of vine and solanaceous crops for a minimum of two years. Good rotation crops include corn and small grains. Avoid planting susceptible crops in poorly-drained soils or in the low areas where there is likely to be standing water after heavy rains. Break up hardpans and plow pans by subsoiling or V-ripping every few years to increase soil drainage. Plant solanaceous crops on domed-shaped (not flat topped), plastic-mulched, raised beds to provide good soil drainage in the plant root zone. Create a waterway system (swales) by ditching low “valleys” and the ends of rows to prevent accumulating standing water between beds and in fields. Clean tillage and cultivation equipment after working infested areas to prevent spreading the disease between fields. Spot-till small areas of infected plants to reduce secondary spread through air borne spores. If available, use resistant varieties if you must plant infested fields.

By: Jude Boucher, Vegetable Crops IPM Program Coordinator, University of Connecticut. Reviewed 2012.

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