Karnal bunt, also called partial bunt, is caused by the fungus Tilletia indica Mitra. Unless the infection is severe, the symptoms are not conspicuous, Jackson says. Symptoms first appear at the soft-dough stage, an early stage of kernel development, as blackened areas surrounding the base of the kernel and extending up along the crease. The fungus usually affects only a portion of the kernel and a few kernels per seed head, according to Gerald Holmes, Imperial County plant pathology farm advisor. One solution to unpalatable grain, says Holmes, is to blend bunted grain with healthy grain so it can't be detected in end products.
Karnal bunt affects common wheat to a lesser degree than durum wheat, triticale and related species.
It is difficult to estimate losses caused by Karnal bunt because it can reduce seed quality, but usually minimally affects grain yield. However, indirect costs are incurred in preventing its spread while losses in marketing opportunities arise because of its presence. Growers are having difficulty selling any wheat grown in the quarantined area, according to Robert Kallenbach, a Riverside County farm advisor. Kallenbach has been assessing risk with Holmes, Jackson, independent statistician Carol J. Adams, UC Berkeley agricultural economist Jerome Siebert and UC Riverside entomologist Thomas Perring for the UC Institute for Desert Agriculture. Heat treatment further reduces the risk to 1 in 5 million.