Downy mildews from gramineaceous crops in North America and those of regulatory concern
C. W. MAGILL (1). (1) Texas A&M University, College Station, TX, U.S.A.
In terms of economic impact<i>, Peronsclerospora sorghi</i> may well be the downy mildew pathogen most threatening to U.S. corn and sorghum production. While optimal reproduction of <i>P. sorghi</i> occurs on susceptible sorghums, the pathogen also infects maize and on rare occasions the disease has spread to corn grown as far north as Indiana. Discovery in south Texas a decade ago of a strain of <i>P. sorghi</i> resistant to metalaxyl seed treatment and subsequently of a new pathotype that reproduces on a formerly resistant commercial sorghum hybrid, raises the possibility that the disease could again lead to significant crop losses. Today, the downy mildew most often observed in maize is ‘crazy top’, caused by <i>Sclerophthora macrospora</i>. Although extra height makes infected plants quite noticeable and the ears are replaced by a mass of proliferating shoots, typically only rare plants in waterlogged areas of a field show symptoms, making disease losses relatively inconsequential. Concern that exotic downy mildews introduced either accidentally or deliberately would severely damage corn production in the U.S. led to two species, <i>P. philippinensis</i> and <i>Sclerophthora rayssiae</i> being placed on the list of plant pathogenic select agents. While sources of resistant maize are available in Asia, the current status of resistance to either of these oomycetous pathogens in popular U.S. cultivars is unknown. Alternate hosts, including sugarcane and sorghum make <i>P. philippinensis</i> of greatest concern.
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