Plant pathogens may be tiny, but they can cause big problems, from stunted roots to rotting fruits. Effective disease management is all about preventing these pathogens from thriving; it's easier to prevent disease than it is to manage it once it gets started. Before plant disease can occur three things are required: the presence of a pathogen can cause disease, a susceptible host, and the right environmental conditions. Taking away any of these can prevent disease.
Causes of disease
A good crop rotation plan that includes a lot of cover crops provides a solid foundation for preventing plant diseases. These oats and peas were planted in the fall after harvest of a cash crop.
Photos by Vern Grubinger.
Plant diseases can result from living pathogens such as viruses, nematodes, bacteria and fungi, or from nonliving (abiotic) factors such as excess heat, cold, air pollution, nutrient imbalance or physical injury. Sometimes there are several factors involved, as when environmental conditions like saturated soils allow infection by pathogens that cause root rots. While there are specific management strategies for individual diseases, there are many general practices that should be used to prevent disease in your crops.
Start with clean plants
Diseases that start early are likely to mean bigger losses in yield and income compared to those that arrive late in the season, after some or most of the potential harvest has been realized. So, always start the season with disease-free seeds and transplants. Use hot water or fungicide-treated seed where appropriate. Note that hot water treatment may reduce seed vigor, so treatment should be done as close to planting as possible. When purchasing plants, be sure to inspect them closely upon arrival for signs of infection, both above ground and below.
Disinfect pots, flats, tools and bench surfaces before planting in the greenhouse. Power wash field equipment prior to the season, and when moving it between fields to the extent possible. Promptly turn under crop debris to bury disease inoculum. Mow or cultivate to keep weeds down in and around fields as these can harbor pathogens. During the growing season, scout fields often and seek out and destroy individual plants that show signs of disease. Sometimes catching the first few affected plants can limit the spread of disease.
Watch the water
Water can also harbor pathogens. Water that drains from fields should be diverted away from greenhouses and irrigation ponds. Recycling water in the greenhouse can spread diseases, especially bacterial infections, so scout frequently for problems.
Optimize crop growth
Apply fertilizer based on soil test recommendations rather than a shotgun approach. Do not plant in poorly drained soils; use subsoiling, raised beds or a rotation with deep-rooted cover crops to help improve drainage before planting. Avoid cultivator blight; adjust equipment and cultivate carefully to minimize injury due to mechanical cultivation. Even small wounds can allow for the entry of disease-causing organisms.
Plant diseases often results from a combination of factors. In onions, environmental stresses such as drought, heat or cold can damage tissue, making it easier for a variety of fungal pathogens to get established and spread.
Avoid susceptible cultivars
Growing varieties that are resistant or tolerant to plant disease is one way to avoid problems. Resistant varieties suppress the activity of a pathogen so there are few if any disease symptoms. Tolerant varieties endure a disease without significant losses in quality or yield although they may exhibit disease symptoms. Obviously, resistant or tolerant varieties are not available for all diseases on all crops. However, it makes sense to plant varieties that can handle the most common pathogens a crop is likely to encounter in your area. If you've had trouble before with a specific disease, seek out resistant varieties to trial on a small scale to determine if they meet your market needs and are suitable for your farm's growing conditions. Lists of resistant varieties for different diseases are on the Cornell site Vegetable MD, at http://vegetablemdonline.ppath.cornell.edu/. Click on the link for resistant varieties on the left.
Manipulate the crop environment
Managing the environmental conditions in a field can go a long way towards preventing plant disease. Installing tile drains or raised beds in poorly drained soils reduces the risk of soil-dwelling diseases that require standing water to spread from plant to plant. Using wide row spacing and staking can enhance air movement, which promotes faster drying and thus slows the development of some foliar diseases. For the same reason, drip irrigation poses less risk of promoting foliar disease than overhead irrigation.
Maintaining healthy soil is another tactic to keep diseases in check. Fertile soil helps plants avoid nutrient deficiencies that can cause stress and make them more prone to infection. For example, tomato plants that are deficient in nitrogen have been shown to be more susceptible to early blight. Avoiding excess fertility is also important. For example, overapplying nitrogen may result in excessive foliage and a thick canopy with limited air movement and thus a crop that is more prone to disease. Regular additions of cover crops may also help keep plants healthy. This promotes an abundance of soil microbes, including beneficial organisms that can help reduce the chances of root diseases getting the upper hand.
This is perhaps the most important cultural practice for disease management, but only for diseases that can survive from year to year. Those that typically arrive from afar, such as late blight or downy mildew of cucurbits, are not managed by rotation. Without a good rotation, the buildup of crop diseases is almost a sure thing. The challenges to good rotation are not only the practical issue of where to put all the crops you need to sell, but also the lack of clarity about exactly what crops should be rotated for how long a period of time in order to suppress disease.
As a rule of thumb, most crops should not be planted more than once every third year on the same ground. Longer is better if possible. Some crops, such as garlic, are vulnerable to diseases that can build up and dramatically reduce production, so these should have a longer than average rotation. Other crops, such as sweet corn, have less disease pressure, especially root diseases, and they may cope with an every-other-year type of rotation.
A good rotation plan not only moves crop families around but alternates with cover crops to help maintain soil health. Regardless of how you make rotation decisions, keep good records to help with your future planning, especially is a disease comes along that requires a change in your traditional rotation in order to suppress it.
Know what's out there
Educate yourself and your crew about the symptoms of common diseases and the need for early detection. Finding out sooner rather than later that a disease has arrived on your farm will greatly improve the chances of effective management. Employees should be on the lookout for disease symptoms, even if they don't know exactly what they find. They should understand the importance of bringing a symptom to your attention so the cause can be identified.
Poor soil drainage can promote a variety of root diseases. Subsoiling, tile drainage, raised beds and rotation with deep rooted cover crops can all help improve this situation.
A great resource for help with disease identification is the color photos in the back of the "New England Vegetable Management Guide," called the "Northeast Vegetable and Strawberry Pest Identification Guide." The guide is available for $25 from the region's state vegetable specialists. The entire pest management supplement is also online at: www.nevegetable.org/index.php/ordering.
Another excellent resource with many color photos of diseases plus life cycle information and management recommendations is the book "Diseases and Pests of Vegetable Crops in Canada." This is available for $58 from the Entomological Society of Canada by calling 613-725-2619 or online at: www.cps-scp.ca/dpvcc.html. For professional identification of diseases, or to confirm your own diagnosis, send a sample to your land grant university's plant diagnostic clinic.
The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.