No. 19/August 1, 1997
Midseason Prospects for Plant Diseases
Midseason diseases should begin to make their appearance within the next 2 weeks. The diseases discussed in this article will not all appear throughout the state but are commonly associated with specific weather conditions and will vary in severity depending on the weather conditions, resistance or tolerance of the plants, and rotation and tillage systems.
SDS, or sudden death syndrome, is seen on scattered soybean plants or in groups of plants throughout a field. It is caused by the fungus Fusarium solani, but only one particular strain of that organism. It is more common in the southern regions of the state but can be seen in the central parts as well. Northern Illinois usually has only scattered areas in a few fields, due to different weather conditions. SDS is most damaging in warm and wet areas, although it seems initially to become active following a period of cooler-than-normal weather. Symptoms usually start to develop after flowering and progress from small yellow spots on the upper leaves to complete death of tissues between the veins (interveinal necrosis). The veins remain green throughout this progression. Plants also may drop leaves or flowers prematurely; and yields can be reduced, depending on the time of infection and the time to maturation of the beans. Root tissues may develop a reddish discoloration near the center of the taproot. Other roots may be rotted, and the surface can be black in color. A diagnostic feature of this disease is the white color of the pith. Brown stem rot, another fungal disease more common in northern Illinois, can have the same leaf symptoms but also has a dark brown and rotted pith area.
White mold or Sclerotinia stem rot. Although there have been few reports of white mold appearing on soybeans, this disease is active in states north of Illinois and should also begin to appear in the northern parts of Illinois within the next couple of weeks. Weather conditions have been favorable for this pathogen during the past few days. The following article appeared in the Indiana Pest and Crop Bulletin by Don Scott and Greg Shaner and provides an excellent overview of white mold.
"White mold is caused by the fungus Sclerotinia sclerotiorum. This fungus can infect a wide range of plants including several crop, vegetable, ornamental, fruit and weed species. Field crops reported as host include alfalfa, beans (green and dry), birdsfoot-trefoil, canola, clover (alsike, crimson, red, etc), oats, peppermint, potato, rye, soybean, sunflower, sweet clover (annual yellow, yellow, white), tomato and wheat. Corn and sorghum are not hosts. Canola, sunflowers and tomatoes are highly susceptible. Some common weeds reported as hosts include bleedingheart, buckhorn, black medic, cocklebur, common chickweed, chickory, chokeberry, crabgrass, curled dock, dandelion, dog fennel, evening primrose, garlic, goldenrod, horsenettle, Jerusalem-artichoke, lambsquarter, milkweed, mullein, mustard (black), oxeye daisy, parsnip, penneycress, peppergrass, pigweed, purslane, prickly lettuce, ragweed, shepherds-purse, thistles, wild sweet potato, velvetleaf, and yellow rocket.
"The symptoms of white mold are frequently overlooked when the disease first occurs and especially the first time it occurs in a field. Initially, only individual plants or small groups of plants prematurely die in late July to late August. The first symptom of white mold appears as a bleached area on infected stems. These lesions are usually three to ten inches above the soil. As the disease progresses upward and downward in the stem, a fluffy, white mold growth develops on the surface of infected stems during wet, humid weather. When stems are girdled by the disease, the plant wilts and dies. The diagnostic sign for the disease is the presence of large (1/16 to 3/4 inch), hard, black, oblong to irregularly shaped sclerotia. The sclerotia develop on stems in the white mold growth, within the pith tissues, and occasionally, in pods. When cut open, the interior portion of a sclerotium is white, gray or pinkish.
"The fungus survives primarily as sclerotia on or in the soil, or as mycelium in infected plant residues. The sclerotia can survive, when buried in the soil, for up to seven years. Sclerotia are highly resistant to chemicals, dry heat up to 158°F, and prolonged freezing and thawing. During wet, cool weather, sclerotia on or very near the soil surface germinate by means of tiny, mushroomlike structures (apothecia). Microscopic spores (ascospores) are produced in the apothecia, and are forcibly ejected from the apothecia with wet conditions. This is the reason that most stem infections occur three to ten inches above the soil line. Prolonged periods of low soil temperatures (4159°F) and high soil moisture are most favorable for apothecial development. The most favorable conditions for ascospore production are 5459°F and wet soils.
"When ascospores are forcibly ejected and land on flowers, stems, branches or pods, infection can occur when these tissues are wet or with high relative humidity. Senescent flowers (shortly after pollination) are the most suitable nutrient base for the fungus in soybeans. Extended periods of cool, moist weather during flowering, and especially when soybeans are flowering in the lower portion of the canopy, are most favorable for disease development. When infection occurs, the fungus develops from the flowers, or other infected tissues, into the main stem where the white mold and sclerotia are produced.
"The disease is more severe where soybeans follow diseased soybeans or other hosts. It is also more severe in narrow row soybeans, due to closure of the canopy earlier in the growing season when soils are usually cooler and wetter. Relative humidity is also higher beneath a closed canopy and plant tissues remain wet for longer periods of time favoring infection by the ascospores. Wisconsin research indicates that irrigation during flowering, and especially early flowering, increases the severity of the disease in infested fields.
"Sclerotia and infected crop debris may be easily spread to non-infested fields by combines or other equipment, surface water, etc. The disease may also be spread through sclerotia or infected crop residues in improperly conditioned seed, or, rarely, in infected seed."
Gray leaf spot (GLS) has begun to make an appearance in many areas of the state. This fungal disease requires high humidity or frequent rains to infect a corn leaf successfully. GLS is most damaging if infections occur on the ear leaf prior to tasseling or on other upper leaves within the 2week period following tasseling. If the disease is not evident at the ear leaf or above following the 2-week period after tasseling, then economic losses are usually very low. Scouting should be under way throughout the state to detect the grayish, rectangular lesions. Because the disease is most commonly transmitted from residues to the new plants, begin scouting by examining the bottom three to four leaves for lesions. Because the pathogen is favored by high humidity, the fungus is most likely to be found on lower leaves first.
Most reports do not indicate that GLS is a major threat at this time. However, the high humidity and scattered showers have produced ideal conditions for sporulation and infection throughout the southern parts of Illinois during the past few days. Because tasseling is well under way through the southern and central parts of the state, GLS damage may not be significant in many fields this year. However, there is ample evidence to suggest that the inoculum is present and that weather conditions are favorable for infection.
H. Walker Kirby, Extension Plant Pathology, (217)333-8414