First author: U.S. Department of Agriculture--Agricultural Research Service, Department of Plant Pathology and second author: Department of Statistics, North Carolina State University, Raleigh 27695.
Deoxynivalenol (DON) concentrations in mature wheat grain are usually correlated with symptoms produced by Gibberella zeae infection. However, there have been numerous observations of unacceptably high DON in asymptomatic crops, which can lead to lower-than-expected milling reductions in DON. We conducted a field experiment with winter wheat to examine the effect of infection timing and postanthesis moisture on grain quality and DON accumulation. Seven to eight soft red winter wheat cultivars were grown in three successive years in a misted nursery in Kinston, NC. Spikes were randomly selected for individual spray inoculation at 0, 10, or 20 days after anthesis (daa). Starting at anthesis, plots were subjected to 0, 10, 20, or 30 days of mist. Inoculated spikes and noninoculated controls were collected at harvest-ripeness, and the threshed grain was assayed for Fusarium-damaged kernels (FDK) and DON. In 2 of 3 years, percentages of FDK were significantly lower from 10-daa infections than from those at 0 daa, although DON concentrations were the same at the two inoculation timings in 2 of the 3 years. Those results indicate that the period of maximum susceptibility to wheat spike infections by G. zeae is close to or slightly less than 10 daa in North Carolina. In 2 of 3 years, FDK--DON correlation was greater for 0- and 10-daa inoculations and for 0- to 20-daa misted treatments than for the later-inoculated or longer-misted treatments, respectively. The percentage of “low-FDK, high DON” (LFHD) observations (defined as FDK ≤ 4.0%, DON ≥ 2 μg g--1) was higher in 2007 than in 2005 or 2006 (41, 14, and 18%, respectively). In both 2006 and 2007, high percentages of LFHD observations (≥60%) occurred under marginal disease conditions involving late infection. We conclude that late infection is an important factor leading to LFHD grain. Periods of rain soon after anthesis likely favor the low-symptom, high-DON scenario, and conditions that create greater within-crop variability of anthesis timing may also be important.