Crazy Top Downy Mildew of Grain Sorghum Observed in Many Delta Fields

Grain sorghum with a twisted top and a zinc deficient appearance may be suffering from crazy top downy mildew instead.

Over the past three weeks, numerous grain sorghum fields have been observed to contain stunted, twisted, and slightly bleached plants.  Most of the affected fields run in a band from north of the Clarksdale airport north into Tunica County.  Diseased plants tend to be in areas where standing water occurred for a prolonged period of time.  Even though these fields appear to have either herbicide injury or a nutrient deficiency, the issue in question is crazy top downy mildew.  The organism that causes the disease is soilborne.  Plants that have been in standing water for an extended period of time or have gone underwater so the submersion covers the top of the whorl can experience crazy top downy mildew.  Even though the majority of infected fields observed to date are in Coahoma and Tunica counties, additional counties may be observed to contain grain sorghum with crazy top downy mildew.

Some confusion may have occurred regarding the terminology on the hybrid information provided by the seed companies.  Two different downy mildews occur in grain sorghum: crazy top downy mildew and sorghum downy mildew.  Commercially available germplasm does not contain resistance to crazy top downy mildew.  Limited resistance within some hybrids to sorghum downy mildew can be obtained.  However, sorghum downy mildew is not the particular disease occurring in Mississippi grain sorghum fields.  Observationally, sorghum downy mildew will produce yellow to white stripes from the base of the leaf to the leaf tip.  But, plants infected with the sorghum downy mildew will not be twisted or stunted.

Crazy top downy mildew.

Grain sorghum fields exhibiting crazy top may green up as fields dry and plants begin to respond to supplemental nutrition.  However, even though the infected plants may appear to recover, the yield loss associated with infected plants could be severe.  In most cases, plants infected by the fungus, and exhibiting crazy top symptoms may not produce a head.  The entire head can usually be replaced by leaf material as a result of infection.  Once infection occurs, no management alternatives, such as a fungicide application, will prevent yield loss associated with the disease.  Be mindful, that in some cases, crazy top downy mildew can be confused with herbicide injury (e.g., an auxin containing product, glyphosate injury) or a nutrient deficiency.  In addition, crazy top downy mildew may mimic, at least observationally, a zinc deficiency.

Management options

Three management strategies exist for grain sorghum fields exhibiting crazy top infection.

1) Do nothing and allow the grain sorghum to reach maturity.

2) Terminate the affected grain sorghum and replant grain sorghum.

3) If atrazine has not been applied to the field, terminate the grain sorghum and plant soybean.