Plant Research Centre, Agriculture Canada
Observations of silk resistance to Fusarium graminearum in germplasm with resistance to Aspergillus flavus and Heliothis zeae
--L.M. Reid, D.E. Mather and R.I. Hamilton
The ear-rotting pathogen Fusarium graminearum Schwabe can enter the maize ear via the silk and/or silk channel. Infection then spreads vertically from the rachis tip. Few sources of resistance to silk infection have been identified. It was hypothesized that genotypes with resistance to other pests that enter the ear via the silk might have some resistance to F. graminearum. Resistance to F. graminearum was therefore evaluated in germplasm known to be resistant to the fungal pathogen Aspergillus flavus Link ex. Fries and/or the corn earworm Heliothis zeae Boddie.
The germplasm evaluated consisted of: two A. flavus resistant lines, Mp313E and Tx601, and three F1 hybrids with the highly resistant inbred line CO272 and the moderately resistant inbred line F7 (Mp313E X F7, Tx601 X F7, and, Tx601 X CO272); six H. zeae resistant germplasm lines (GT113, GT114, GT115, GT117, GT1118, and GT119) released by Widstrom et al. (Crop Sci. 28:202, 1988); and the photo-insensitive Mexican landrace Zapalote Chico whose silks contain the flavone C-glycoside maysin which has been associated with earworm resistance (Waiss et al., J. Econ. Entomol. 72:256-258, 1979).
Twenty plants of each inbred, hybrid, or population were inoculated by injecting 2ml of a spore suspension of F. graminearum (2 X 106 spores/ml) into the upper silk channel of the primary ear approximately 6 days after silk emergence. Resistance to spread of infection was evaluated in mid-October using a 7-class disease rating scale where 1= no infection, 2= 1-3%, 3= 4-10%, 4= 11-25%, 5= 26-50%, 6= 51-75%, and 7= >76% of kernels infected.
An in vitro test of detached silk tissue (Naik and Busch, Can. J. Bot. 56:1113-1117, 1978) was used to evaluate the ability of F. graminearum to degrade and assimilate silk of different genotypes. From each of 10 primary ears, two 1g samples of a cross-section of the silk mass were cut from the silk channel. Each sample was placed in a petri dish containing filter paper moistened with 3ml of sterile water. One sample of each pair was sprayed with 3ml of spore suspension (5 X 105 spores/ml) and the other with 3ml of sterile water. The pairs of petri dishes were incubated in a growth cabinet for 7 days at 25 C and 95% relative humidity. Samples were dried in an oven at 80 C for 48 hrs and weighed. Reduction in silk dry weight due to degradation and assimilation by the fungus was measured as the percentage difference in dry weight between the inoculated sample and the water-control sample.
Three of the H. zeae resistant lines (GT115, GT117, GT118), the two A. flavus resistant lines, and the hybrids Tx601 X F7 and Tx601 X CO272 were highly resistant (mean disease rating <3.0) (Table 1). The hybrid Mp313E X F7 and the landrace Zapalote Chico were moderately resistant (mean disease rating 3-4). The remaining three H. zeae resistant lines were as susceptible as the F. graminearum susceptible inbred CO266.
In some of the Southern U.S. genotypes, there was very little fungal degradation and assimilation of the silk tissue. There was less reduction in dry weight for silk tissue from Mp313E, Tx601, Mp313E X F7, and Zapalote Chico than for silk tissue from the highly resistant inbred CO272.
Some of the Southern U.S. genotypes evaluated warrant further study as sources of resistance to Fusarium ear rot. Four inbreds (GT117, GT118, Mp313E, and Tx601) had lower disease ratings than the highly resistant inbred CO272. However, only GT117 was adapted to the northern-temperate region.
Table 1. Evaluations of resistance to F. graminearum and days
to silking of 11 inbreds, 3 hybrids, and one landrace grown at Ottawa,
|Mean Disease Ratinga||Tissue Degradation (% Reduction in Dry Wt.b)||Days to Silking|
|GT113||5.2 ab||29.0 a||82|
|GT114||5.0 a||24.2 a||81|
|GT115||2.1 d||8.1 c||123|
|GT117||1.5 d||9.3 bc||89|
|GT118||1.1 d||9.6 b||124|
|GT119||5.2 a||26.3 a||84|
|Mp313E||1.0 d||1.7 d||124|
|Tx601||1.6 d||1.3 d||123|
|Mp313E X F7||3.1 bc||3.1 d||85|
|Tx601 X F7||2.9 bc||9.7 b||82|
|Tx601 X CO272||2.6 cd||9.2 bc||83|
|Zapalote Chico||3.0 b||3.5 d||82|
|CO272||1.9 d||7.2 c||85|
|F7||3.2 ab||23.9 a||69|
|CO266||5.0 a||32.1 a||79|
bMeans followed by the same letter are not significantly different at the 0.05 level by Duncan's multiple range test.
Within Zapalote Chico, some plants had disease ratings of 6 or 7 but most had disease ratings of 1 or 2. This may reflect genetic segregation for resistance, and Zapalote Chico may be a more useful source of resistance than would be expected from its moderate disease rating.
Because of the high frequency of resistance observed among the genotypes
evaluated, further pathological and phytochemical studies are being conducted
to determine whether there is a relationship among the mechanisms against
graminearum, A. flavus, and H. zeae.
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