Diseases can have a significant effect on production of specialty seed crops. Seed growers must pay attention to diseases that affect the vegetative growth stage of the crop, as well as those that affect the reproductive growth stages (flowering and seed formation). Some diseases, such as Verticillium wilt of spinach, become symptomatic only when the crop enters the reproductive stage; these diseases are more important to seed growers than to vegetable growers
Diseases can have a significant effect on production of specialty seed crops. Seed growers must pay attention to diseases that affect the vegetative growth stage of the crop, as well as those that affect the reproductive growth stages (flowering and seed formation). Some diseases, such as Verticillium wilt of spinach, become symptomatic only when the crop enters the reproductive stage; these diseases are more important to seed growers than to vegetable growers (unless the vegetable crop also has a flowering stage, e.g., tomato or potato). While vegetable growers are concerned primarily with the pathogens that affect marketable yield and quality, seed growers must also learn how to diagnose and manage seedborne pathogens and the microorganisms that affect seed quality. Pathogens usually remain viable for longer in seed than in vegetative parts of the plant or in the soil. Seeds are a major means of survival of some plant pathogens and of introducing new pathogens to a field or region.
Disease management tactics are either preventive (actions taken to avoid or reduce the likelihood of disease problems) or curative (treatments that eliminate or reduce the effects of a particular disease after it has become established). Because there are few effective curative practices available to organic farmers, organic farmers focus their disease management efforts primarily on preventive cultural practices. Such practices include planting pathogen-free seed, planting in fields of low inoculum potential and in locations with good air movement, adopting wide row spacing, orienting the crop rows to maximize air movement between rows, and tying or staking seed crops to improve air circulation and reduce humidity in the canopy. If feasible, consider using drip or furrow irrigation instead of overhead irrigation, or irrigate earlier in the day to allow the canopy to dry before nightfall.
Some significant pathogens of seed crops are soilborne, such as Fusarium wilt of spinach. To manage soilborne pathogens, it is important to know the cropping history of the field and to adopt appropriate crop rotations. A rotation of 6 to 15 years, depending on the susceptibility of the spinach cultivar, is required to control Fusarium wilt in spinach seed crops. Some soilborne pathogens affect more than one crop, e.g., the fungus that causes Verticillium wilt of spinach can also infect potato, so it is important to avoid growing other crops in the rotation that may be alternative hosts to soilborne pathogens that affect the seed crop.
Strict management of, and screening for, seedborne pathogens of vegetable crops is critical to maintaining high seed quality. Even low levels of seed contamination can cause epidemics of some diseases when infected seed is planted in the field. For example, the tolerance level for contamination of crucifer seed with the causal agent of black rot, Xanthomonas campestris pv. campestris, is 0 contaminated seeds in 10,000 to 50,000 seeds (depending on the market or country in which the seed will be distributed).
Seeds contaminated with a pathogen can be treated physically (e.g. hot water) or chemically (e.g. bleach) to destroy inoculum or reduce the incidence of infection. Some physical and chemical treatments may reduce seed quality (germination, vigor, and/or longevity), so it may be important to test a particular seed treatment on a small sample of seed and check for possible phytotoxicity to the seed before treating an entire seed lot. Hot water treatment can only be used on some crops, such as brassicas, carrots, tomatoes, peppers, and lettuce, but even on those crops very precise parameters must be followed for hot water treatment to avoid damaging the seed. There are a number of biological and natural disease management products coming onto the market that are approved for use on organic farms, but it must be noted that the efficacy of these biocontrol products may vary among sites, crops, and diseases, reflecting the the complexities and particulars of interactions amongst the host, pathogen and environment. Therefore, planting pathogen-free seed, when possible, is always preferable to trying to eradicate a pathogen from seed.
When growing seed crops, the following steps will minimize the risk of disease:
- Learn to diagnose and manage the diseases of each crop grown.
- Know what seedborne pathogens are important to your crop and prevalent in your region, and ensure, through communication with your seed supplier or contractor, that the seed you are planting has been tested to be pathogen-free or has been treated preventatively.
- Design and manage the cropping system to minimize the likelihood of disease development. See the related article Keys to Disease Management in Organic Seed Crops.
- Scout fields regularly for early symptoms of disease development.
- If you must apply materials to your crops to control disease, communicate with your certifier to make sure the materials are permitted for use on organic farms and labeled for that crop and disease. See the related article Can I Use this Product for Disease Management on my Organic Farm?
- Screen seed lots for seedborne pathogens before sale. This is the responsibility of the seed company, and all diagnostic laboratory results should be communicated to the seed grower. Be aware that there may not be commercial seed health tests available for the particular crop of interest, particularly specialty crops grown on a small scale and for which there has been limited research on seedborne pathogens.r