Spot blotch is a leaf disease of wheat caused by Cochliobolus sativus. Cochliobolus sativus also infects other plant parts and in conjunction with other pathogens causes common root rot and black point.
Introduction Foliar blight, Helminthosporium leaf blight (HLB), or foliar blight has been a major disease of wheat (Triticum aestivum L.) worldwide. Foliar blight disease complex consists of spot blotch and tan spot. Spot blotch is favored in warmer environments, whereas tan spot is favored in cooler environments such as United States. The tan spot forms of foliar blight appears in United States causing significant yield loss. With changed climatic conditions the disease is supposed to be increasing in cooler parts of the world. Among foliar blights the tan spot, caused by Pyrenophora tritici-repentis, is the most destructive leaf spot disease found in all wheat classes throughout the growing season across North Dakota. The spot blotch form of foliar blight is severe particularly in warmer growing areas characterized by an average temperature in the coolest month above 17 °C. In the past 20 years, HLB has been recognized as the major disease constraint to wheat cultivation in the warmer eastern plains of South Asia. About 25 million hectares of nontraditional wheat growing area are under the pressure of the disease. See: Knowing the Enemy: Foliar Blight
Symptoms Early lesions are characterized by small, dark brown lesions 1 to 2 mm long without chlorotic margin. In susceptible genotypes, these lesions extend very quickly in oval to elongated blotches, light brown to dark brown in colour. They may reach several centimetres before coalescing and inducing the death of the leaf. Fruiting structures develop readily under humid conditions and are generally easily observed on old lesions. If spikelets are affected, it can result in shrivelled grain and black point, a dark staining of the embryo end of the seed. The small dark brown spots on the leaves contrast with the larger, light brown spots or blotches produced by tan spot and septoria avenae blotch. typical spot blotch sysmtom
Crop losses In recent years, Helminthosporium leaf blights (HLB), caused by both Cochliobolus sativus and Pyrenophora tritici-repentis, have emerged as serious concerns for wheat cultivation in the developing world. The disease causes significant yield losses overall 22% to complete failure of crop under severe epidemics.
Distribution The first observation in Zambia was made in the 2017/8 rainy season by someone from the Zambia Agricultural Research Institute.
Control measures The disease is very serious in different parts of the world. The management of this disease requires an integrative approach.
An integrated approach The best way to control Helminthosporium diseases is through an integrated approach. It includes the use of a variety of resistance sources, such as hexaploid wheat from Brazil and China (some of which is rate-limiting), alien genes and synthetic wheats. In addition, appropriate management practices that enhance the health of the plant populations, in general, are critical. Cooperation of pathologists, breeders and agronomists will be necessary to ensure sustainable control of this group of diseases. Economic feasibility of recommended practices has to be determined as part of the research. Options for controlling tan spot and spot blotch include disease-free seed, seed treatment with fungicides, proper crop rotation and fertilization, cultural practices in order to reduce inoculum sources, the use of chemicals and the research of disease resistance. The latter offers the best long-term control at no cost for the farmer and is ecologically safe.
Seed health In Brazil, it is recommended not to plant seed lots with more than 3% black point to limit spot blotch. Seed treatment may prove to be appropriate, although the inoculum remaining on secondary hosts or in the soil may reduce the treatment efficiency. Seed treatments with phytoalexin inducer appeared to provide good protection to wheat seedlings against B. sorokiniana infection. Seed treatment with fungicide will help protect germinating seed and seedlings from fungi causing seedling blights. Fungicide seed treatments include: captan, mancozeb, maneb, thiram, pentachloronitrobenzene (PCNB) or carboxin guazatine plus, iprodione and triadimefon (Stack and McMullen, 1988; Mehta, 1993). Seed-borne inoculum of P. tritici-repentis can be controlled with seed-applied fungicides, such as guazatine and guazatine + imazalil, but other chemicals are also effective.
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