Stem rot is prevalent throughout the rape growing areas. It is most destructive during prolonged periods of rainfall because both the production of the spore-bearing phase and the development of subsequent infections are favored by cool moist conditions. Incidence of the disease varies greatly from year to year. Within individual fields, yield loss varies with the percentage of plants infected and the stage of growth when infection occurs. Losses are most severe when infection occurs at midflowering and rarely exceed 15 to -20%.
Sclerotinia stem rot has been the most serious disease of canola in North Dakota and Minnesota, with average incidence (percent infected plants) as high as 19 percent in North Dakota in 1993 and 19 percent in Minnesota in 1997. Estimated state-wide losses from Sclerotinia were as high as 13 percent in North Dakota (1993) and 13 percent in Minnesota (1997). In severely infected fields losses were estimated as high as 50 percent.
Sclerotinia diseases probably affect most, if not all, annual vegetables, ornamentals, and field crops and cause huge amounts of losses both in the field and postharvest. The symptoms caused by Sclerotinia vary somewhat with the host or host part affected and with the environmental conditions. Sclerotinia diseases are known under a variety of names, such as cottony rot, white mold, watery soft rot, stem rot, drop, crown rot, and blossom blight, among others.
Infected plants are most conspicuous( 显著的 ) in the field when the crop is fully podded. The diseased plants become straw-colored and form a striking contrast to the adjacent healthy green plants. diseased plants remain more erect than healthy plants, which tend to lodge from the weight of filling pods. Infections begin as a soft, watery rot on leaves and stems.
When a lesion completely girdles( 包围 ) a stem, the plant wilts and dies. The infected area dries and at maturity is distinctly white, often with zonate markings. The fungus forms hard black sclerotia in the hollow center of the discolored areas of infected stems. The diseased tissue tends to shred, releasing new sclerotia, which either fall to the ground or are harvested with the crop.
Sclerotinia stem rot develops late in the season, with the first visual symptoms appearing by the end of flowering. Dead and lodging plants occur singly or in patches in infected fields. Infections of individual plants usually develop around cast( 脱落物 ) petals( 花瓣 ). The infections may produce a target pattern of light brown, mushy tissues. Infections may spread from infected leaf petioles( 叶柄 ) or branches to larger stems. Infected areas eventually become bleached or white and the tissues become shredded( 破裂、破 碎 ).
If the main stem is infected, plants may die early, reducing seed production. Hard black bodies which resemble rat droppings may be produced in infected stems. These are known as sclerotia. They are helpful in identifying Sclerotinia, but may not be present in every infected stem.
Sclerotinia sclerotiorum Host range: A vast range of broad-leaved plant species including field peas, white beans, alfalfa, red clover, lentils( 小扁豆 ), carrots, potatoes, and sunflower are susceptible to the same disease organism.
The fungus survives as sclerotia in the soil, or as mycelium in debris from diseased plants. In debris new sclerotia are produced as long as the fungus is actively growing. Sclerotia may germinate as mycelium to infect plants directly, as in sunflower, or they may germinate to form tiny mushroom-like apothecia. Apothecia forcibly eject spores which become air- borne and have the potential to infect plants. In the field, apothecia are continuously produced from early June until late September if moisture and temperature conditions are favorable.
The mid- to late-flowering stage of the crop is the most critical period for infection. Later infections result in relatively little damage to the crop. New sclerotia are formed in diseased plant tissue. The tissue disintegrates （碎裂） before or during harvest, releasing sclerotia into the soil and thus assuring continued survival of the fungus. All rapeseed cultivars currently grown are susceptible to stem blight.
When susceptible crops are grown in close rotation a high incidence of infection often results. Cereal crops are not affected and afford a reduction of viable sclerotia in the soil, partly through decay by other organisms in the soil, and partly by permitting sclerotia to expend themselves by germinating in the absence of susceptible host tissue.
However, in fields with a history of stem blight, even a 5-year absence of susceptible crops usually is not adequate to reduce the number of sclerotia in the soil sufficiently to avoid serious levels of infection. The control of Sclerotinia diseases depends on a number of cultural practices and on chemical sprays.
Susceptible crops should be planted only in well-drained soils, the plants should not be planted too close together for air drainage, and the soil should be kept free of weeds between crops. Good control of the Sclerotinia disease has been obtained by spraying the soil or the plants with appropriate fungicides before and during their stage of susceptibility to the pathogen.
INTRODUCTION Our experience over the past few years suggest that SMV does not generally affect yields. The major concern about this disease is seed quality. However, soybean plants are commonly infected with more than one virus, which increases risk of yield loss and lower seed quality. SMV and AMV often occur together in the same plant. Tobacco streak virus and Bean pod mottle virus (BPMV) have also been found in multiple infections.Bean pod mottle virus (BPMV)
SYMPTOM Common leaf symptoms of SMV are a mosaic of light and dark green areas, chlorosis, rough leaves, and leaf curl.The youngest and most rapidly growing leaves show the most symptoms, especially at cooler temperatures. Plant stunting, reduced pod numbers and seed discoloration are also symptoms of SMV infection.
PATHOGEN Soybean mosaic virus (SMV) and Alfalfa mosaic virus (AMV) are virus diseases of soybean that are transmitted by the soybean aphid as the aphids feed on plant sap. General symptoms of SMV and AMV infection are plant stunting, leaf distortion and mottling, reduced pod numbers and seed discoloration.
DISEASE CYCLE Infected seed is the most important way that SMV is introduced into a soybean field. Once the virus is in the field, aphids can spread it from plant to plant as they feed. Over 30 species of aphids transmit SMV worldwide. Recent research confirmed the soybean aphid, (Aphis glycines ) is a vector of soybean mosaic virus. soybean aphid Another concern about SMV is dual infection with other viruses, a common situation that increases the risk of yield loss and reduced seed quality. SMV and Alfalfa Mosaic Virus often occur together in the same plant.
CONTROL Seed transmission of SMV depends on variety and ranges from 0-5% in most modern soybean varieties. Most commercial soybean varieties are susceptible to SMV. However, resistance to SMV has been identified in soybean genotypes and varieties, and it is likely that recommendations for SMV resistant soybean varieties will be available to growers in the near future.
CONTROL Insecticides are not considered effective in reducing transmission of SMV by aphids. Aphids present at spraying are killed, but the field is quickly recolonized by winged aphids and virus transmission can resume. Aphids that contact insecticide residues on the leaf surface are killed, but are still capable of virus transmission prior to death.