The New Agriculture Network's on-line newsletter with seasonal advice for field crop and vegetable growers interested in organic agriculture.

Vol. 1, No. 10 - August 26, 2004

In this issue

	What to expect for late season diseases
	Vegetable disease management
	We'd like your opinion
	Reports from organic growers. Update from Iowa on August 31.

Next issue posted September 9. Read previous issues through our calendar of issues.

What to expect for late season diseases
Darin M. Eastburn, Department of Crop Sciences, University of Illinois at Urbana Champaign

This is the time of year when many diseases of crop plants become most evident, and there are a number of reasons for this based on plant disease cycles, crop development stages, and the environment. Depending on the type of disease, the parts of the plant affected, and the stage of crop growth, late season diseases can result in major reductions in crop yield and quality or have very little impact on the harvested product. There is often very little that can be done in the way of "rescue treatments" to manage plant diseases at this point in the season, but knowing which diseases are present and understanding their potential impact will help growers take appropriate actions and better plan for future disease problems.

One reason that plant diseases are most severe at this time of the season is that pathogen levels have gradually built up over the course of the season. Some plant pathogens infect a plant, produce new inoculum (fungal spores, bacterial cells, virus particles) and spread the disease to neighboring plants during the growing season. Plant pathologists call these polycyclic diseases, because they go through several disease cycles, from initial infection to the production of new inoculum, in a single season. Diseases such as common rust on corn, downy mildew of soybeans or early blight of tomatoes, typically start on a few plants in the planting. When conditions are favorable, the pathogens multiply and spread to other plants in the field, creating an epidemic.

For these types of diseases, management strategies need to focus on preventing inoculum production and plant-to-plant spread. This might be done through using resistant varieties, modifying microclimates by adjusting plant spacing or irrigation schedules, or applying chemicals such as copper sulfate for organic growers. Many foliar diseases of plants (those that infect the leaves, fruit and stems) are polycyclic in nature, and a single infected plant can lead to a significant epidemic if environmental conditions and time allow. So by this time in the season these diseases have had the chance to multiply and spread, and the number of plants showing symptoms continues to increase.

In contrast to polycyclic diseases, monocyclic diseases do not typically spread from plant to plant within the season. Instead, the pathogens that cause these diseases infect a plant during the season, and then produce inoculum which will overwinter and initiate the disease again in seasons to follow. Many of the soilborne plant diseases, such as Rhizoctonia root rot, soybean cyst or Verticillium wilt, fall into the monocyclic category. Because there is no plant-to-plant spread during the season, the amount of disease is determined, in part, by the amount of pathogen present at the beginning of the season. There is usually no dramatic increase in the amount of disease observed as the season progresses. However, some of these diseases can appear to increase dramatically at this time of the season because the environment or plant growth stages favor the expression of disease symptoms.

One example of this is sudden death syndrome on soybeans. This disease is cause by a soilborne fungus, which actually infects the plants fairly early in the season. The pathogen slowly colonizes the soybean plant's root system during the growing season. Few, if any, foliar symptoms are evident before the middle to end of August on soybeans grown in Illinois, when changes in the plant physiology and/or soil environment cause the typical interveinal chlorosis symptoms to develop over a period of a few days. Symptomatic soybean plants are usually found in localized areas within a field because the fungus is in the soil at these locations. The patches enlarge from year to year as the spores of the fungus are spread through soil by tillage and other processes. Because there is no plant-to-plant spread during the season, management practices for monocyclic diseases usually focus on reducing the amount of pathogen present at the beginning of the season. Practices such as crop rotation, soil solarization or some kinds of cover cropping can accomplish this. Of course, resistant varieties will be useful for managing these diseases as well.

Changes in crop maturity also can lead to increased disease levels as the end of the season approaches. As with the soybean sudden death syndrome example, some diseases express symptoms more readily when the plants or plant parts reach a certain stage of development. Another good example of this is anthracnose on tomato fruit. Anthracnose produces symptoms that look as if someone has pushed their thumb into the flesh of a tomato fruit leaving a slight depression. With time a small dark area will develop in the center of that depressed area. These lesions will only develop on red fruit. The fungus can actually infect green fruit, but the symptoms will only develop when the fruit is mature. This can be a problem for growers picking symptomless green fruit, in that symptoms may develop by the time that the fruit gets to market. Crop growth stage also can affect disease development by changing the microclimate of the crop canopy. White mold on green beans and soybeans usually starts to develop when the crop canopy closes over. This pathogen requires high humidity levels to infect, and high humidity levels are easier to achieve in a closed canopy.

Late season disease levels can be influenced by climatic conditions as well. The fungus that causes common rust of corn usually arrives in Illinois sometime between late-June and mid-July. While some infections occur at this time, the disease often does not really take off until mid- to late-August. Hot summer temperatures slow the development of the disease, but when temperatures start to cool in mid-August, the disease can rapidly spread through a growing area.

Other diseases that typically show up later in the season are the powdery mildews and downy mildews. The fungus that causes powdery mildew of cucurbit crops (squash, pumpkins, cucumbers, etc.) does not overwinter in this area, but has to be blown up from the south on air currents each year. The maturing crop and favorable environmental conditions allow the powdery mildew fungus to rapidly spread through a cucurbit crop in mid- to late-season.

Sometimes these late-season diseases can result in significant crop losses, such as the development of black rot on cabbage and broccoli, or brown rot on peaches. With other diseases they may develop after the crop has been set and have no appreciable effect on yield amounts or quality. And sometimes late season diseases can actually hasten crop maturation and improve crop yields. A few years ago I was investigating various strategies for the control of powdery mildew on processing pumpkins. In my studies I found that complete suppression of the disease actually led to lower yields on some varieties, because many of the fruit stayed green too long. In plots with a moderate level of mildew the fruit matured more uniformly and yields were higher. This may be a rare instance, but it can occur.

Late season diseases can be problematic, and they can be frustrating for growers because there is often nothing that can be done to control them after they begin to appear. Good record keeping, understanding what diseases are likely to occur later in the season, and the timely implementation of disease management strategies, often at or before the time of planting, can help reduce the losses to these diseases.

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Vegetable disease management

Dan S. Egel, Southwest Purdue Agricultural Center

Crop rotation
Many disease-causing microorganisms (pathogens) survive in plant debris. When one plants a specific crop year after year in the same soil, these pathogens can build up to high levels. An example would be early blight of tomato. Crop rotation allows the plant debris to decay and the associated pathogens to die out. Some pathogens are soil borne and survive in the absence of plant debris. An example would be Fusarium wilt of watermelon. Managing these pathogens often requires longer crop rotations.

Crop rotation in Table 1 refers to the number of years that the field is planted to a different crop. Growers should rotate from the crop in question to a crop in another plant family. For example, after growing tomatoes, one should avoid tomatoes, potatoes, peppers and eggplants for the length of the rotation. Whether crop rotation is effective or not and the length of crop rotation recommended are listed under each crop/disease combination.

Tillage
Decay of plant debris can be hastened by tilling the debris into the soil. The sooner the plant debris is buried, the sooner decay starts and the pathogens die out. Therefore, it is often wise to till a field in the fall. The deeper the debris is buried, the better. Moldboard plowing may be advisable. To avoid soil erosion, a cover crop can be planted over the winter. Alternatively, some growers may want to remove plant debris and compost it. In such cases it is critical to remove roots as well as tops and make sure the compost becomes hot enough to kill the plant pathogens that may be present. Whether tillage is likely to be effective or not is listed under each crop/disease combination (Table 1).

Resistant varieties
If available, resistant varieties are an excellent disease management option. Varieties with complete resistance to a particular disease will show no symptoms of the disease. Varieties with partial resistance will have symptoms of the disease but measurably less crop damage than a susceptible variety. Partial resistance may not be sufficient to stop disease loss completely, but can be effective when used with other disease management options. Varieties with tolerance may have the same amount of disease as a susceptible variety but sustain less yield loss. Growers should note that some varieties are resistant to only certain races of a pathogen. Unfortunately, resistant varieties are not available for all situations. In addition, varieties that are resistant may lack desirable horticultural characteristics or consumer preference. The availability of resistant varieties is listed under each crop/disease combination (Table 1).

Seedborne
Some diseases may be carried on the seed. In this manner, a disease may manifest itself very early in the season. Resulting disease loss can be great. In addition, some diseases may be introduced into a field through seeds.

A number of seedborne diseases are listed in Table 1. Do not assume that all vegetable seed is contaminated with disease. In addition, most of the diseases that are listed as seedborne in Table 1 can also survive on plant residue. Bacterial canker, for example, may be introduced on tomato seed or may survive on plant residue from a previous year.

Determining whether a particular disease has originated with the seed can be difficult if not impossible. Growers who use transplants should carefully monitor greenhouse production. Growers should scout for diseases at all stages of production.

Saving seed from one year to the next can be rewarding. However, growers should be aware of potential problems. Seed harvested from a fruit with even minor symptoms of disease may be contaminated with pathogens. In addition to plant disease problems, saved seed may not have the characteristics of the original variety. Saving vegetable seeds requires special skills and equipment.

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Northern Illinois
A major change in our weather pattern has occurred this week. For the first time since late June, we are just beginning to experience an extended period of average or above average temperatures along with decent rain and mild evenings. Corn and soybeans are still far behind in development. I finished planting cover crops on August 16 and am finally getting back to spending time in the shop and sorting through the piles of paperwork that have accumulated over the past two months at my desk. In the next two weeks, I will be pulling weeds from bean fields for the second time.

Central Illinois
Finally, summer has returned-this should help summer crops. It has been dry here the last two weeks, but it looks like that should change soon. We are still spraying Bt on peppers for European corn borer. We planted our last outside transplanting this week. Still have a planting of spinach planned and five more weeks of lettuce planting. In the next two weeks, we will plant hairy vetch, clover and rye. Greenhouse clean up should begin next week.

Southern Illinois
The cool spells continue to slow development of corn and soybeans. During the last three weeks, we have received no rain and the ground has dried out significantly. The crops have very shallow roots and this has also stressed them some. Current farming practices include, finished baling stubble hay, combined red clover seed and cutting weeds out of the beans. In the next two weeks, I will be foliar feeding my alfalfa.

Questions: Has anyone tried cleaning wheat over a gravity table in hopes of removing the lighter kernels, which are the kernels that are infected with vomitoxin? If so, have you been able to significantly lower vomitoxin levels? Editor's note: One of our readers, Dr. Bill Wilcke at the University of Minnesota, submitted the following comments:

In years when there are high scab levels in Minnesota wheat, gravity tables are used to remove the light kernels, which improves test weight and reduces vomitoxin levels. Cleaning grain on a gravity table is slow and expensive, but it does help improve the quality. I don't have first hand experience with operating a gravity table, but I was involved in a research project where we compared quality levels and storability of cleaned and uncleaned wheat that had scab infection.

For details see: "Storability of Wheat Infected with Fusarium Head Blight", by WF. Wilcke et al., 1999, Transactions of the ASAE 42(3): 733-742.

Eastern Michigan
Rob Malcomnson reports that soils are starting to dry out. "By the weekend, we could use a shower." Heat is still needed to finish out corn and soybeans. Currently they are clipping wheat stubble and clover, and working on machine repair. In the next two weeks they will be cutting more second cut hay and repairing drain tile. They are also cleaning wheat of vomitoxin.

Central Indiana
Dave Swaim reports that soybeans and corn in the area planted early are starting to senesce. Early harvest is expected. One of the major concerns is that the earliest planted beans, those planted in late March through early April, are showing lots of sudden death syndrome. The third cutting of hay has been done. Tomatoes are showing lots of leaf problems. The pasture situation is quite good.

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