Downy Mildew or Blue Mold (fungus - Peronospora effusa): The downy mildew fungus first causes yellowish areas on the upper side of the leaf. The underneath side of the leaf is marked by a gray to violet-gray fungal growth mat that bears sporangia. The entire leaf is killed on susceptible varieties under optimum environmental conditions. The fungus overwinters in living spinach plants and in the seed. The fungus spores require surface moisture for development. Optimum temperature is around 48 degrees F for germination and 54 to 60 degrees F for development. This disease can be controlled by the use of varieties resistant to races 1, 2 and 3.
Anthracnose (fungus - Colletotrichum spinaciae): Anthracnose shows up as small, dark olive colored spots. As the spots enlarge, they become tan in color. As the lesions coalesce, they kill the entire leaf. During periods of favorable disease development, the foliage appears as if the crop will be lost. With a change in weather, the diseased foliage drops off and the healthy foliage goes on to produce a crop. The fungus overwinters in seed and in crop refuse. Fungicides used in a white rust control will also control anthracnose.
Leaf Spots (fungi - Cercospora beticola, Heterosporium variable): These are minor diseases of spinach which may cause damage in some years. Cercospora leaf spots are white and, usually, small in size. Extended periods of precipitation and high humidity may allow spots to become large and even coalesce. Protective type fungicides offer some control. Ridomil does not control Cercospora.
Heterosporium leaf spots are larger and have a greenish black fungal growth on both sides of the leaf as the disease develops. Control measures are not generally recommended.
Fusarium Decline (fungus - Fusarium oxysporum f. sp. spinaciae): Plants are subject to infection at any stage of development. Infected plants turn yellow and wilt. The lower, older foliage is affected first. Plants which are infected are either stunted or killed. Infected plants lose their feeder roots and the vascular system of the taproot is darkened. The fungus is seed borne and can live in the soil indefinitely. Fortunately, white rust resistant varieties also have Fusarium resistance.
Blight (virus): Cucumber mosaic virus attacks spinach causing plants to become faintly chlorotic. Chlorosis increases until the entire plant becomes yellow. Crown leaves are narrow, wrinkled and develop an inward rolling of the margins. Plants are stunted. Death may occur in some severely infected plants. The virus is aphid transmitted. Most commercial varieties are resistant.
Seedling Disease (fungi - Fusarium spp., Rhizoctonia spp., Pythium spp., Others): Preemergence and postemergence damping off can seriously reduce stands. Rotate with corn, plant after soils have cooled down in the late summer or fall, use a fungicide seed treatment and buy fresh, high quality seed.
Tobacco Ringspot (virus): First symptoms are small, indistinct, chlorotic spots which appear on the young foliage. These may coalesce to form large yellow areas. In advanced stages, leaves take on a copper bronze chlorosis. There is no malformation of the foliage. Affected plants rarely die but remain yellow and stunted. No control measures are known.
Beet Curly-Top (virus): Infected plants are marked by a rosette of tightly curled, small leaves in the center of the plant. As the disease develops, the growing point is killed and the plant dies. The virus is transmitted by beet leafhoppers.
Aster Yellows (mycoplasma-like organism): (See Aster Yellows.)
False Root Knot (nematode - Naccobbus crucifera): Plants are stunted; in cases of severe early infection, death will occur. The root system of infected plants is characterized by large galls covered with numerous rootlets. This nematode is particularly damaging on spinach plants in the fall. This nematode does little damage in hot soils.
Root Knot (nematode - Meloidogyne sp.): (See Root Knot)
Stress (abiotic - heat and/or saturated soils): The quality of spinach can decrease quickly following stress. Yellowing occurs within a few days after high temperatures and/or flooding. Choose planting dates carefully for your areas and provide good drainage.
Scurf (fungus - Monilochaetes infuscans): Scurf is also known as soil stain. It is most severe in wet or poorly drained soils. Scurf appears as light brown areas on the outside of the roots and are only skin deep. These areas may be small or run together to form large, irregular patches. Although superficial, infection reduces grade and causes undue shrinkage in storage. Avoid use of scurfy seed potatoes. Treat sweet potato roots with fungicide before planting. Practice rotations of three to five years. Avoid planting where organic matter in the soil is not well decomposed. See also entry at NCSU.
Root Knot (nematode - Meloidogyne sp.): Root cracking is often associated with severe root knot infection. Pitting and other surface blemishes are also noted. The cracking is sometimes confused with growth cracks. The splitting of small whip-like roots with small black surface lesions will reveal tiny, pearly-like nematodes embedded around the blackened areas. These also occur in mature potato flesh but are more easily viewed for diagnostic purposes in smaller roots. Vines may be stunted and show nutritional deficiencies. Root knot can be controlled by selecting only healthy seed roots, free of root knot nematodes. These should be planted in a bedding site that is also free of the nematode and preferably has not been used in many years. Preplant nematicides or soil fumigants will help control field infestations. Most newer varieties such as Jewel, Cordner and Topaz have some degree of resistance.
Stem Rot (fungus - Fusarium oxysporum f. sp. batatas): Young leaves at the tips of vines turn yellow while older leaves wilt then drop so that the center of the hill becomes bare. Infested slips may die soon after setting or become stunted and yellow. Stems at the soil line may turn slightly blue (blue-stem). The inner stem portion at or below the soil line becomes discolored with brown streaks in the vascular system. Use of stem cuttings rather than slips or sprouts for planting stock will reduce the likelihood of new infections in the field. Control by using resistant or tolerant varieties. Selection of seed roots free of the disease is difficult because external symptoms are not always present. Limited control may be obtained when treating seed with a fungicide for other soilborne diseases.
Black Rot (fungus - Ceratocystis fimbriata): Circular, almost black spots appear on the sweet potato. Affected and adjacent areas have a bitter taste. Small, black lesions often completely girdle underground stems. The causal organism fruits abundantly in storage and spores are easily spread by mites so that a few diseased roots may result in severe storage losses. Select seed potatoes carefully at harvest time and again at bedding. Use new soil each season for bedding seed. Cull all slips that appear sickly and those with black spots on lower stems. Crop rotations, clean plant beds and use of a fungicide on planting seed will also aid in controlling black rot. If black rot occurs in storage, the rotted roots should be destroyed and the storage area disinfected.
Soil Rot or Pox (fungus - Streptomyces ipomoea): Potatoes are often mis- shapen with rough, scabby pits or shallow surface lesions that result in scars. Soil rot is most severe in soils approaching alkalinity. Control may be obtained by soil fumigation. Some varieties have tolerance to soil rot. See also entry at NCSU..
Foot Rot (fungus - Plenodomus destruens): Foot rot is not as widespread as black rot or stem rot and, therefore, losses to this disease are not as great. On infected vines, the base of the stem turns brown at the soil line and leaves nearest the crown turn yellow and drop. It is most commonly observed from mid-season to harvest. Individual plants may produce few potatoes even though large vines develop during the season. The potatoes that are produced develop a firm brown rot at the stem end. Seldom is the entire root affected. Controls recommended for black rot and scurf will also control this disease. If foot rot is recognized to be a serious problem, then early harvest will aid in reducing losses.
Internal Cork (virus): Foliar symptoms vary from purple ring spots to vein-clearing. Symptoms become masked as leaves grow older. The most characteristic symptom is seen within potatoes since no visible surface symptoms are evident. Slicing a potato lengthwise will reveal brownish to black corky areas inside. If seed potatoes are stored at 60oF., development of the corky areas does not proceed. However, if seed are stored at 70 degrees F., severe corkiness develops. Storing seed potatoes at 70 degrees F. is one way of checking for this disease. The variety Puerto Rico is highly susceptible. Internal cork should not be confused with chilling damage.
Storage Rots (fungi - soft rot - Rhizopus stolonifer, Java black rot - Botryodiplodia theobromae, black rot - Ceratocystis fimbriata, charcoal rot - Macrophomina phaseolina and other): Storage rot losses are greatly reduced when disease control practices are followed that yield high quality sweet potatoes from the field. Some fungi causing storage rots infect roots in the field before harvest while others enter the potatoes through wounds made at harvest or during handling. Decayed spots may be dry or soft, the latter due principally to Rhizopus rot. Sweet potatoes should be cured for seven to 10 days at 85 degrees F. and at 90 percent relative humidity before being stored. Curing allows a natural healing process to take place. Potatoes should be stored at 50 to 60 degrees F. and at 85 to 90 percent relative humidity. Avoid rough handling between curing and storage because additional wounding may occur. Store in a thoroughly clean and disinfected room with adequate ventilation. Do not permit the storage temperature to drop below 55 degrees F. or injury will occur. For control of Rhizopus rot during the processing-to-market interval, apply a fungicide spray immediately after washing, when potatoes are on conveyor belts or rollers.
Cold Damage (physiological): Pithy, dark discolorations will appear internally on sweet potatoes subjected to low temperatures. Temperatures do not have to reach the freezing point to cause damage. Sweet potato is a tropical plant and will suffer injury at temperatures below 55 degrees F.
(physiological): Scientists have not yet determined the conditions that cause cracking of sweet potatoes. In some varieties, it appears to be an inherited characteristic. Rotation reduces incidence of growth cracks.
Other Diseases: Southern blight caused by Sclerotium rolfsii (common in plant beds), cotton root rot caused by Phymatotrichum omnivorum and root rot caused by Rhizoctonia solani and charcoal rot caused by Macrophomina phaseolina.