Reaction of waxy and non-waxy maize inbreds infected with Fusarium
--Chungu, C and Mather, DE
Fusarium graminearum Schwabe, the asexual state of Gibberella zeae (Schw.) Petch causes ear rot of maize in most maize growing areas in the world. The pathogen penetrates ears by growth of the mycelia down the silks to the kernels or through wounds made by insects or birds. The characteristic symptom of the disease is a pink to reddish coloration on the surface of infected kernels and husks.
Warren (Phytopathol.68:1331-1335, 1978) observed that some opaque-2 maize inbreds were more susceptible to F. moniliforme ear rot than their normal-endosperm counterparts. Similar observations were reported by Reid et al. (Can. J. Plant Sci. 72:915-923, 1992) with F. graminearum. Opaque-2 kernels tend to be softer, which may allow pathogens to penetrate the kernels easily.
Waxy maize differs from normal dent maize in that its endosperm starch is 100% amylopectin whereas that of normal maize is composed of 75% amylopectin and 25% amylose. This difference is important to the manufacturers of food and industrial products. According to Coe et al. (Corn and Corn Improvement, p142, 1988), the waxy kernel type displays uniform marble-like opacity and has kernel hardness similar to that of normal kernels. Little is known about the relative resistance of waxy inbreds and their non-waxy counterparts to ear rot caused by F. graminearum. The objective of this study was to compare the responses of waxy and non-waxy inbreds to F. graminearum.
An experiment was conducted at Ste-Anne-de-Bellevue (Quebec, Canada) in 1993 (the experiment was seeded again in 1994, but failed due to poor germination). Eleven waxy and non-waxy inbreds (seed provided by David Bauté from MaizeX, Ontario and R.I. Hamilton, RI, Plant Research Centre, Ottawa) were planted in a split-plot design with four replications. Inbreds were randomized as main-plot units and two inoculation methods (silk channel injection and a kernel-stab technique) as subplots. Individual ears were inoculated by: (a) injecting 2 ml of the macroconidial suspension in the centre of the silk channel seven days after silk emergence, and (b) by inoculating the ears using a kernel-stab technique, 15 days after silk emergence. In the latter technique, a probe consisting of four nails (1.5 cm) fixed to a cylindrical wooden handle was dipped into inoculum and then used to stab through the husk to wound three to four kernels in the middle of the ear. Primary ears of the waxy inbreds were bagged before silking to avoid contamination with pollen from their normal counterparts and were later hand-pollinated. Inoculated ears were harvested in mid-October and disease severity assessed by rating the percentage of rotted area using a 7-class kernel rating scale where 1= no symptoms present, 2=1-3%, 3=4-10%, 4=11-25%, 5=26-50%, 6=51-75%, and 7=76-100% of the kernels infected. Disease incidence was calculated as the percentage of ears with severity rating of 2 or greater. Data were analyzed using the general linear models analysis of variance, and mean comparisons were performed using Duncan's multiple range test.
Effects due to inbreds were significant (P<0.05) for both disease incidence and severity (Table 1). Differences between the two inoculation methods were significant only for disease severity.
Table 1. Mean values for disease severity and incidence in waxy and
non-waxy inbred lines with inoculation techniques at Ste-Anne-de-Bellevue
Disease incidence values were high for both waxy and non-waxy inbreds. Most inbreds exhibited high disease severity with both inoculation methods. Three inbreds, A641wx, LH82 and Mo17wx had only moderate disease severity after silk-channel injection. However, these inbreds were all susceptible with the kernel-stab method. One inbred, Mo17wx, exhibited lower disease severity than its normal counterpart. It appears that most of the inbreds evaluated in this study do not have sufficient resistance in the silk and kernels to slow or inhibit the spread of ear rot.
To avoid pollen contamination, the ears of the waxy inbreds were bagged prior to silking and the bags remained on the ears four weeks postinoculation. The environmental conditions within the bags could have influenced the spread of ear rot on the waxy inbreds. Enerson and Hunter (Can. J. Plant Sci. 60:1123-1128, 1980) found increased colonization intensity in ears inoculated with a toothpick and bagged for 35 to 63 days. In contrast, Sutton and Baliko (Can. J. Plant Pathol. 3:26-32, 1981) found that bagging after inoculation suppressed the growth of F. graminearum. In this study, it was not possible to determine the effect bagging had on disease development.
This study showed that the inbreds differed in their reaction to infection by F. graminearum when the silk channel method was used, however, none of the waxy inbreds differed from their non-waxy counterparts. No significant difference was observed among inbreds when inoculum was directly applied to the kernels. We did not find any evidence that the waxy endosperm trait confers ear rot resistance or susceptibility. However, our comparisons of waxy vs. non-waxy lines were confounded by the fact that ears of the waxy lines were bagged to prevent pollen contamination.
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