Potential losses due to “minor diseases” should not be underestimated. For the purpose of this discussion, minor refers to diseases that are relatively non-destructive under most conditions, or, they are destructive but infrequently encountered.
Damping-off, Root Rot and Crown Rot by Rhizoctonia and Pythium. Rhizoctonia solani is a soil-borne fungus that is common in greenhouses and widely distributed in field soils. It has a broad host range. It is an aggressive pathogen to young plants and a minor pathogen to older plants. Transplants that develop cankers in the greenhouse may perform poorly or not survive when planted into the field. Rhizoctonia survives indefinitely in soils by colonizing organic material and producing sclerotia. It is not found in soilless growing media but it is common in greenhouses, on floors and on the ground. When contamination occurs to the growing medium, Rhizoctonia rapidly colonizes the soil and causes root or crown rot. Leaves that touch the ground may also become infected.
Like Rhizoctonia, Pythium has a broad host range. Pythium is often referred to as a water-mold because of its propensity to grow in water, and because it has unique swimming spores. Pythium survives indefinitely in soil by producing oospores. The oospores are stimulated to infect by germinating seeds and root exudates. High soil moisture and high fertility generally increase disease severity. Depending on the species of Pythium, high or low temperatures may be favored. Like Rhizoctonia, Pythium is soil-borne and is more of a problem in the greenhouse than in the field. In the field, Pythium can be a problem in wet soils, especially to transplants.
Management of Pythium and Rhizoctonia should start in the greenhouse. Use a soilless medium to grow transplants. However, if accidental contamination occurs, the soilless medium will generally be very conducive to growth of Pythium and Rhizoctonia. Use new plug frays, or clean and disinfect trays or pots if they have been used. Be careful not to contaminate the growing medium with dirty hands, tools, or hose nozzles. After watering, hose ends should be hung up to keep them off the floor. Keep the humidity low and maintain adequate but not excessive fertility. Avoid planting into fields with a history of Pythium or Rhizoctonia diseases.
Sclerotinia Blight. Sclerotinia blight is caused by the fungus Sclerotinia sclerotiorum. Sclerotinia blight is not commonly a problem in the greenhouse but because the fungus is soil-borne it is a perennial problem wherever it becomes established. The pathogen produces sclerotia inside or on the surface of lesions. Sclerotia are hard, black structures from 1/16th to 1/2 inch in length. They can survive for years in the soil and are responsible for initiating disease. Germination of sclerotia and initiation of disease are dependent on prevailing weather conditions. Optimum temperature for sclerotia to germinate is about 52o F but some sclerotia germinate over a wide range of temperatures. Germination is also dependant on continuous wetness for 10 days. Only sclerotia that are within about an inch of the soil surface germinate. Upon germination they form apothecia, tiny cup-shaped mushroom-like structures. A single apothecium will produce more than 1 million spores which can travel in the wind for several 100 yards.
Management of Sclerotinia blight relies primarily on crop rotation. Many vegetable crops and weeds are susceptible to this fungus; corn and grasses are not. Lettuce, cabbage, tomato, carrot and snap beans are among the most susceptible and should not be grown on land known to be contaminated with Sclerotinia. Rotation away from susceptible crops should be for 7 years. A single infected head of cabbage may produce more than l000 sclerotia. Removal of diseased plant material from the field is recommended.
Anthracnose. This disease is caused by the fungus Colletotrichum coccodes and it can also occur in the Solanaceae and the Cucurbitaceae. The two most important hosts are tomato and potato. Pepper and eggplant are less frequently affected. On peppers, disease is most prevalent on fruit that is left on the plant for long periods of time, although, unripe fruit can develop latent infections. Anthracnose is more common on red peppers that have a long ripening period. The disease appears as soft sunken lesions on the fruit. On red peppers, the lesions may bleach the color from the fruit. In some cases the lesions become brown and then black from the formation of setae and sclerotia. Or, the center of the lesions may develop pustules of salmon-colored spores which break through the epidermis in wet masses.
Rotate away from solanaceous plants for at least 2 years. Choose planting sites that have good drainage. Control weeds. Some resistant cultivars are available. Pepper cultivars that have a shorter ripening period may escape anthracnose. For late-maturing peppers, where there has been a history of disease, apply fungicide several weeks before harvest.
Alternaria Leaf Spot and Fruit Rot. Alternaria solani causes early blight of tomato and potato, which are very destructive diseases; however, pepper is not an important host. Alternaria solani occasionally causes a minor leaf spot on pepper foliage. Alternaria alternata may cause fruit rot, particularly following sun scald or blossom end rot. Sun scald on pepper fruit usually occurs when the foliage is sparse and the peppers are exposed to sunlight. The injury becomes tan and shrunken and may appear water-soaked. When Alternaria colonizes these lesions they become chocolate brown to black and the fungus may be evident by a felty, dark brown to black growth. Alternaria fruit rot may also occur post-harvest.
To prevent Alternaria rot, choose cultivars that shade fruit well. Control diseases such as bacterial spot that may defoliate the plant. Provide adequate moisture and calcium to prevent blossom end rot. Application of fungicides are not warranted for this disease.
Bacterial Soft Rot. Bacterial soft rot is cause by Erwinia. This bacterium is commonly associated with plants of all kinds and can be cultured from soil and surface water. Fields previously cropped to potato or cabbage are reported to have higher populations of Erwinia in the soil. Bacterial soft rot is primarily a post-harvest problem in the northeast, except when fruit is injured by insects. Bacterial contamination begins in the field and washing the fruit can spread the bacterium and provide it with easy access to wounds, especially at the stem-end. The stem offers a natural wound by which the bacterium can enter the fruit. Fruit infected by Erwinia develops a slimy rot. The rot spreads quickly and within several days the entire fruit may collapse from rot.
There are no chemical controls that can be used in the field to prevent Erwinia soft rot from developing. However, the bacterium can only enter the fruit through a wound so care should be taken during harvesting and washing. Peppers picked during the heat of the day should not be subjected to cold water since this would allow water and surface contaminants like Erwinia to be drawn into the fruit.
Nematodes. Root-knot, lesion and stubby root nematodes occasionally cause problems in peppers. If populations of any one of these nematodes are high, significant injury can occur. Of the three, root-knot nematode, Meloidogyne hapla is the most destructive to pepper as well as other vegetable crops. Root-knot is also the easiest nematode problem to recognize because they cause galls on the roots.
The lesion nematode, Pratylenchus penetrans, is widely distributed in agricultural and nonagricultural soils. In agricultural soils, potatoes and strawberries are favorite hosts and populations tend to increase on them. Lesion nematodes have a wide host range which in addition to most vegetable crops, includes grasses and weeds. When populations of lesion nematodes are high, plants may appear stunted, may wilt during the heat of the day, and yield may be reduced. Infected roots may be underdeveloped, especially fine roots, and on close examination small lesions may be evident. The lesions provide avenues for root infecting fungi such as Rhizoctonia.
The stubby root nematode, Paratrichodorus minor, can infect grasses, corn, and vegetable crops. The stubby root nematode feeds on the surface of the root, at the root tip, and along the sides. It does not enter into the root like root-knot and lesion nematodes but penetrates the outer layer of cells with a grooved spear. With high populations, above-ground symptoms include stunting and poor vigor. Root systems are reduced and become course, or stubby with extensive short branches and necrosis.
Crop rotation can be a useful management tool but the wide host range of these nematodes make it impractical in many cases. In any case, it is important to have suspected nematode problems confirmed by a soil and/or root tissue assay. If populations are above 100 nematodes/100 cc of soil, it is very likely that they are causing significant injury to the crop.
Preplant fumigation is an option and is very effective for reducing nematode populations, but may not be economical. Peppers vary in their susceptibility to root-knot nematode. The USDA has recently released two highly resistant cultivars; Charleston Belle and Carolina Wonder. Nematode-suppressive green manures, including Sudangrass and rapeseeds may reduce populations of these nematodes, and their damage.
Marigolds, Tagetes patula T. erecta will reduce populations of root-knot and lesion nematodes when interplanted or used in rotation but this would not be economical for most growers. A new marigold cultivar, Polynema, has recently been developed specifically for reducing nematodes in infested soil. Liberal introduction of organic matter into soil will introduce nematode-pathogenic fungi, and will reduce nematodes but will not provide significant control when a highly susceptible crop is planted into nematode-infested soil.
By: Robert L. Wick, Department of Microbiology, Morrill Science Center, N-203, University of Massachusetts, Amherst, MA 01003 Tel. (413) 545- 1045 email: rwick@pltpath.umass.edu
Originally published: Proceedings. 1999. New England Vegetable & Berry Growers Conference and Trade Show, Sturbridge, MA. p.101-103.
Reviewed by: T. Jude Boucher, IPM, University of Connecticut. 2012
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