Latent Infection of Euphorbia lathyris and Weeds by Macrophomina phaseolina and Propagule Populations in Arizona Field Soil. D. J. Young, Department of Plant Pathology, University of Arizona, Tucson 85721. S. M. Alcorn, Department of Plant Pathology, University of Arizona, Tucson 85721. Plant Dis. 68:587-589. Accepted for publication 19 January 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-587.

Infection of roots of Euphorbia lathyris by Macrophomina phaseolina occurred within 1 mo of an October 1980 planting in southern Arizona, but disease symptoms were not evident until May 1981. Propagule populations ranged from fewer than 1 to 246 sclerotia per gram of Gila silt loam, the lowest density sufficient to cause the death of more than 90% of field-grown plants in August 1981. The highest density of sclerotia occurred during a second consecutive cropping of E. lathyris in one field. Sclerotia of M. phaseolina survived in untilled soil for 1 yr. Six new weed hosts of M. phaseolina that were detected are Amaranthus palmeri, E. hyssopifolia, E. prostrata, Ipomea coccinea, Sonchus oleraceus, and Tidestromia languinosa.

Keyword(s): charcoal rot.