Field reactions of maize varieties commonly grown in Australia to boil smut caused by Ustilago zeae

The reactions of 62 hybrid maize varieties and 24 inbred lines to infection by boil smut (Ustilago zeae) were studied in the field in the 1982-83 and 1983-84 seasons. Plants were artificially inoculated and the incidence of disease (DI) was assessed on the plant as a whole and at particular loci (ears, axillary buds above and below the ears, tassels and leaves). A disease severity rating (DSR) was developed based on the mass of galls at various loci relative to plant mass. Of the 86 hybrids and inbreds tested, inbred 25, with a high DI overall and severe disease at all infection loci, was the only variety found highly susceptible to disease. Nevertheless, significant (P < 0.05) differences in susceptibility were detected among other hybrids and inbreds. Thirty-six of the major hybrid varieties were classified by cluster analysis into those with low overall DI (20 hybrids averaging 1.4%), moderate DI (12 hybrids, 5.0%) or high DI (4 hybrids, 9.8%). Hybrids with high DI included GH128, GH390, QK694 and Hycorn 1. Two DSR classes were apparent, high (11.0) and low (6.1). No hybrid had both high DI and high DSR. The hybrids General and PX75 had moderate DI and high DSR, while 9 hybrids had high DSR with low DI. Insufficient data were available for a similar classification of the remaining 26 hybrids. Hybrid E368 had a high DI. Among 13 inbred lines, 12 occurred in a low DI class (average 2.3%) and one in a high DI class (A691N, 14.6%). Only one low DSR class was apparent. Insufficient data were available for a similar classification of the remaining I I inbreds, but GN7, HBc and 25 had high DI, while Y 1 had virtually no disease. Disease incidences at most of the infection loci were highly correlated within varieties, except between ear and tassel or leaf incidences, and leaf and tassel incidences, for which no correlation was apparent. No positive correlation could be found between husk cover of mature ears and boil smut incidence in the ears. Except for the locally selected inbred 25, hybrids and inbreds developed in Australia from old open pollinated varieties had about the same disease incidence and severity as material introduced from the U.S.A. over the last 20 years. An evaluation of screening methods indicated that whorl inoculation increased DI on average only from 0.8 to 2.2%. Disease incidence decreased progressively as sowing date was delayed from November to February.