First Report of Charcoal Rot (Macrophomina phaseolina) on Sunflower in North and South Dakota.

Abstract

In September 1998, symptoms suggestive of charcoal rot were observed on oilseed sunflower (Helianthus annuus L.) plants in western North and South Dakota. Symptoms first observed on plants approaching physiological maturity consisted of silver-gray lesions girdling the stem at the soil line, premature plant death, and reduced head diameter. The pith in the lower stem was completely absent or compressed into horizontal layers. Black, spherical microsclerotia were observed in the pith of the lower stem, underneath the epidermis, and on the exterior of the taproot. Confirmation of Macrophomina phaseolina (Tossi) Goid. as the causal agent was based on the size of the microsclerotia, which ranged from 80 to 90 μm in diameter, from both infected sunflowers and pure cultures (1). The only other sunflower pathogen known to form microsclerotia is Verticillium dahliae Kleb., whose microsclerotia are irregular in shape and are 15 to 50 μm in diameter. Some prematurely dead sunflower plants lacked typical charcoal rot stem lesions, but contained Macrophomina microsclerotia. Plants with atypical symptoms were colonized by the sunflower stem weevil (Cylindrocopturus adspersus (LeConte)) and the black sunflower stem weevil (Apion occidentale Fall). This agrees with observations in Texas, where Macrophomina-infected sunflower plants parasitized by stem-feeding insects often displayed atypical charcoal rot symptoms (3). Charcoal rot incidence in 1998 in western North Dakota was 25%, compared with 0% in eastern North Dakota. Charcoal rot was not observed in 1999, the fourth wettest growing season on record, but was observed again in 2000 and 2001. The recent increase in sunflower production in western North and South Dakota, areas typically hotter and drier than the eastern portions of both states, and the potential involvement of stem weevils as vectors of Macrophomina (2) may lead to an increased incidence of charcoal rot in sunflower.