Incidence of Phoma terrestris, Causal Agent of Red Root Rot, in Corn Roots and Soil in Delaware and on the Eastern Shore of Maryland.

Abstract

Red root rot of corn (Zea mays L.) has caused sporadic disease losses in Delmarva during the past 6 years. A survey was made between 21 August and 14 September 1995 in 11 counties in Delaware and on the eastern shore of Maryland to detect Phoma terrestris (E. M. Hans.) in corn roots and in the surrounding soil. Sampling was based on one field per 1,200 ha of corn production in each county. Three root samples and one soil sample were collected from each of 100 corn production fields from corn plants with diseased basal stalk tissue. The presence of P. terrestris was confirmed by cultures from root samples that produced a beet-red color on Watson's sterile wheat straw agar (2). Serial dilutions of soil samples in 0.1% water agar were mixed with unsolidified Watson's basal medium, followed by amendment with a 2-(4-thiazolyl)benzimidazole solution (1). P. terrestris was detected in root samples from 99% of the fields surveyed and from 82% of the soil samples. Soil populations ranged from 0.2 to 200 CFU per g of soil, with a mean of 12.5 CFU. This survey indicated that P. terrestris is widespread in corn production fields in Delaware and on the eastern shore of Maryland. Disease symptoms in surveyed fields indicated that pathogenicity on corn is variable, generally does not result in serious losses, and is related to hybrid susceptibility and growing conditions.