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  A Publication of UVM Extension's Vermont Vegetable and Berry Program

Organic Vegetable Disease Management

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Plant pathogens may be tiny, but they can cause big problems, from rotting fruit to wilting and dying plants. Disease management is all about prevention – it’s easier to stop a disease from getting started than it is to manage it once it starts spreading.

It takes three to tango. Diseases are like a kitchen stool--to stand on their own, they need three legs: a susceptible host, the presence of an infectious agent (a pathogen), and the right environmental conditions. Practices aimed at preventing crop diseases usually focus one of these legs at a time, but going after all three isn’t a bad idea!

Disease-prevention practices are much the same for conventional and organic growers. The main difference is in the menu of materials that can be applied to crops to protect against disease. And, since organic farmers are supposed to use organic pesticides as a last resort, they need to do a really good job with preventative cultural practices.

Don’t host the party. 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. But it makes sense to plant varieties that can handle at least one or two of the pathogens a crop is likely to encounter in your area. If you’ve had trouble before with a specific disease, by all means 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.

Lock the doors.  It pays to keep pathogens off your farm, or to stop them from spreading to new fields or greenhouses if they are already present. There are many ways to exclude pathogens from your production system. First, try to keep out any materials that are already diseased or that might harbor inoculum that can cause infection.

Since many diseases can be seedborne, clean seed is a must. Not all diseases can be tested for, but when available, be sure to purchase seed that has been certified to be disease-free, or close to it.  It’s not really possible to assure that an entire seed lot is absolutely free of pathogens, but certified seed gives assurance that the risk posed by a seedborne pathogen is low.

Whether you buy transplants grow your own, inspect them frequently to make sure they are as free of visible disease symptoms before they go in the field. If a disease starts to show up, it may make sense to treat seedlings while they are still in the greenhouse instead of waiting until they are in the field and the problem gets worse.

When producing transplants, all materials should be as free of pathogens as possible. Use new pots and trays, or reuse them only if they are thoroughly cleaned with steam, bleach, or other disinfectants. Soilless potting mix can prevent introduction of soilborne pathogens into the greenhouse, but recycling old potting mix can negate that benefit.

Moving soil around on the farm is a good way to spread disease. Take steps to limit outside soil from entering the greenhouse on shoes, wind and equipment. Even a small amount of soil adhering to tractors or implements can spread soilborne pathogens from infested fields into clean fields. It’s a good idea to power-wash equipment when moving between fields, especially those that are separated by significant distance.

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.

Removal, or rouging, of infected plants can help slow or prevent the spread of disease, particularly if it’s only on a few plants. With some diseases like Phytophthora blight on tomato, pepper or squash, incorporating infected rows or entire fields of plants is recommended to minimize further spread.

At the end of the season, sanitation is still important. Turn under old plants and crop residues immediately after harvest to slow the development of disease and limit the potential for over-wintering of inoculum.

Stop ‘em in their tracks.  By selecting or manipulating environmental conditions, it is possible to limit or prevent plant disease. A site that has well-drained soil reduces the risk of soil-dwelling diseases that require standing water to spread from plant to plant. Using wide row spacing can enhance air movement, which promotes faster drying and thus slows the development of some foliar diseases

Building healthy soils that contain an abundance of beneficial organisms is another tactic to keep disease in check. Fertile soil also helps plants avoid nutrient deficiencies that can cause stress and make them more prone to infection. In some cases excess fertility can cause a problem. For example, too much nitrogen may result in excessive, succulent leaf growth that is more susceptible to disease.

Adding compost to soil can benefit fertility and tilth, and it may also benefit disease management in some cases. Because compost is such a variable material, it is difficult to give specific recommendations about how, or if, it will suppress disease. In general, it appears that well-made, mature composts are most desirable for this purpose.

When preparing fields, tillage should reduce plant residues left from previous crops.
Raised beds and/or subsoiling should be used to address drainage problems. Seed and seedlings should be planted at the proper depth and provided with adequate moisture to promote rapid emergence and establishment.

As the crop grows, irrigation management is very important to disease control. Regardless of the system you use, the goal should be to satisfy the water needs of your crops without putting on excess water that can encourage pathogenic fungi in the soil. For preventing most foliar diseases, drip irrigation is preferable to overhead sprinkler irrigation because it does not wet the foliage and promote disease development. When using overhead irrigation try to time the application of water so foliage can dry rapidly.

Crop rotation is perhaps the most important cultural practice for disease management, but it may also be the most complicated. It’s not always clear exactly what crops should be rotated for how long a period of time in order to suppress a disease. And besides that, growers still have to meet their market demands and plans for efficient use of equipment and labor.  However, it is clear that crop rotation helps keep many diseases from building up over time – and that keeps bad things from happening.

A good rotation plan moves crop families around and alternates cover crops with cash crops. Some growers have a systematic rotation plan, and for others rotation is a seat-of-the-pants affair, guided by current conditions and past experience. Regardless of how you make rotation decisions, keep good records to help with your future planning.

Spray and pray.  If you have reason to expect a disease to appear, as when late blight is in the neighborhood, or if scouting turns up the first signs of a disease like powdery mildew, then it may be necessary to apply a protectant material to prevent crop loss.

Quite a few copper and sulfur-based fungicides are allowed for organic use, and they are relatively inexpensive and pose minimal environmental risk. Copper fungicides have some activity against a wide range of fungi and bacteria, but their effectiveness is limited, and under high disease pressure probably will not provide excellent control. Sulfur products also provide some control of many pathogens, but they usually are excellent for managing only some diseases such as powdery mildew. Both copper and sulfur can burn sensitive crops.

Organically-approved bicarbonate fungicides have recently become available. Bicarbonates have demonstrated god activity against powdery mildew and a few other diseases. Peroxide-type materials are also available for general suppression of diseases on plant surfaces.

Other organic options for disease suppression are products that contain microbes or their by-products, including species of Trichoderma, Bacillus, and other beneficial organisms. For the best results possible with these materials, proper application and timing are essential. Most materials do not perform well if the disease is established, so applications should be made prior to infection.

Know the enemy, and its location. Whatever kind of farming you do, it helps to know what diseases you’re up against. Some diseases are easy to identify, since they have tell-tale symptoms: concentric rings of dead tissue are characteristic of early blight on tomato and potato, for example. But positive identification of most plant diseases requires help from a university or commercial laboratory with specialized equipment and trained personnel. A key step toward effective disease management is sending samples to such a lab at the very first sign of disease symptoms.

When you identify a disease problem on the farm, be sure to put it in your records. That way, a few years down the road, you’ll still know exactly which field to avoid with susceptible crops.

Published: February 2005
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