Helminthosporium leaf blights: spot blotch and tan spot

In recent years, Helminthosporium leaf blights (HLB), caused by Bipolaris sorokiniana and Pyrenophora tritici-repentis, have emerged as serious concerns for wheat cultivation in the developing world. Due to the increasing food demand and to limited agricultural land, wheat is grown in more intensive cropping systems - an example is the rice-wheat rotation covering 12 million ha in South Asia. Intensive cultivation and cultivation in marginal land has increased severity of HLB.

Yield losses due to HLBs are variable, most commonly 20-100 percent have been reported and in several areas it is the major biotic constraint that hampers growing wheat as a commercial crop. Spot blotch is more prevalent in warm areas whereas Tan spot was ranked as the most economically important wheat disease in cooler climated like North Dakota, United States.
Causal organisms-
Spot blotch - Cochliobolus sativus (Helminthosporium sativum) Syn. Bipolaris sorokiniana, Drechslera sorokiniana
Tanspot - Pyrenophora trichostoma (Helminthhosporium tritici-repentis) Syn. P. tritici-repentis, Drechslera tritici-repentis

Symptoms
Spot blotch - Lesions caused by this disease are elongated to oval in shape and are generally a dark brown color. As lesions mature, the centers often turn a light brown to tan color, surrounded by an irregular dark brown ring

Tan spot - At first, lesions appear as tan to brown flecks, which expand into large, irregular, oval- or lens-shaped tan blotches with a yellow or chlorotic margin. As these spots coalesce, large blotches are formed. The development of a dark brown to black spot in the center of the lesion is characteristic of the disease. As the disease progresses, entire leaves, spikes, and even whole plants may be killed.

Spot blotch affects wheat, triticale, barley, and most grasses. It is found worldwide, but is especially prevalent in more humid and higher rainfall areas. Similarly, tan spot can affect wheat and several related grasses; triticale, barley, and rye are less frequently affected. The disease is found in the major temperate wheat-growing areas.

If infection occurs early in the crop cycle and conditions remain favorable for development, complete defoliation is possible; major reductions in yield and severely shrivelled kernels will then results.

Story of fight against HLB in South Asia
Farmers began to grow wheat in warm flat lands (Gangetic plains) as new CIMMYT -wheat genotypes (green revolution wheat) became available. Due to effective resistance leaf rust become less problematic, spot blotch emerged as second generation disease. With global warming, the disease severity has been increasing year-by-year, studies done in South Asian region has shown this.


CIMMYT pathologist Dr. Etienne Duveiller and Whear Breeder Ram C. Sharma have found an effective method to select for resistance: finding wheat with a heavy grain weight, early maturity, and resistance to both pathogens. Wheat that carries these three traits together makes for wheat with higher resistance. Through regional collaborative trials in South Asia, they have bred and identified wheat lines that look promising. While better than anything previously seen in the area, these wheats can still suffer up to 35% yield losses—and have a huge impact on resource poor farmers who grow their wheat for food, as most do in Nepal. Significant efforts has also been done in collaborating partners in South Asian region.
When the temperature soars to 26-28°C, however, no wheat can resist the disease. This is why it is so important to find wheat that matures early to avoid the abrupt rise in temperature accompanied by hot winds in late March and April. This becomes difficult as most farmers in the region are delayed planting wheat as they wait for their rice harvest to finish and the paddies to dry up.
In addition to genetic resistance, solutions can come in the form of good management. Surface seeding, when seed is broadcast on the mud directly after the rice harvest, allows earlier planting and gives the wheat crop a jump start on the heat. Studies done has emphasized combined selection for heat stress tolerance and HLB resistance / tolerance. Crop rotation and soil nutrients are important because healthy soils help the crop resist the disease. Also, Duveiller and Sharma have found that wheat is better able to withstand the disease with proper soil moisture.

Some of promising resistant / moderately resistant genotypes:
Milan/Shanghai #7, Chirya.1, and Chirya.7, Chirya 3 (=NL 750), SW89.5422, Yangmai #6, Ning 8201, K 8027, SW89-5193, SW89-3060, SW89-5422, Ning 8319, NL781, Croc 1/A. sq.// Borl 95, G162, Yangmai-6, NL 785, , Sabuf, HLB 19, NL 868, BL 1740, Jinmai 4058, Ning 8201, K 8027, K 7, WH 542, Mayoor (HLB 48), PRL/Toni, Longmai-10

References for further details:

Wikipedia - spot blotch (wheat)
http://en.wikipedia.org/wiki/Spot_blotch_(wheat)

Duveiller E, Dubin HJ (2002) Helminthosporium leaf blights: spot blotch and tan spot In: Curtis BC, Rajaram S, Gómez Macpherson H. Bread wheat: improvement and production. FAO Plant Production and Protection Series No. 30. Food and Agriculture Organization of The United Nations, Rome.
web: http://www.fao.org/docrep/006/y4011e/y4011e0k.htm

Knowing enemy of wheat http://www.cimmyt.org/english/wps/news/2005/nov/knowingEnemy.htm