L.Gilchrist, C. Velazquez, and J. Crossa


Introduction

The expression of wheat host plant resistance to head blight caused by Fusarium graminearum Schw. varies widely, depending on environmental conditions (rainfall, temperature) and the inoculum used (age, concentration, incremental substrate, and isolates). It is important to have good control of these factors to avoid variation in the expression of resistance.

At present, a mixture of highly virulent pathogen isolates is commonly used as inoculum in screening wheat for Fusarium head blight (FHB) resistance in the belief that there are no vertical races in F.graminearum, as noted in the literature. There are, nonetheless, significant differences in pathogenicity among isolates that can greatly influence the measurement of resistance levels (Mesterhazy, 1997).

In our program differences in pathogenicity observed during FHB resistance evaluation made us suspect there was significant cultivar x isolate interaction. This led us to initiate the study reported here, whose main objective was to evaluate and confirm the presence of cultivar x isolate interaction.

 

Materials and Methods

Fig. 1. Geographical areas affected by fusarium head blight in Mexico where the isolates were collected

During the 2000 crop cycle in Atizapan, Toluca, Mexico, a trial was carried out in which four resistant (Sumai # 3, Frontana, Catbird, and Sha4/Chilero) and one susceptible (Flycatcher) wheat cultivars were inoculated with eight different F. graminearum isolates. The test isolates originated in Tepatitlan (isolates 3, 4, 5, 6), Jesus Maria (2), and El Tigre (1) in the state of Jalisco, and in Patzcuaro (7, 8), state of Michoacan. (Fig. 1)

The trial was planted with three replications; the cultivar was the main plot and the isolate, the sub-plot. The inoculum was increased in mungo bean medium, and its concentration adjusted to 50,000 spores/ml after growing five days. Twenty wheat spikes per plot were inoculated at flowering using the cotton method (Gilchrist et al., 1997).

Supplementary moisture in the form of mist irrigation was provided on the four rainless days. The different treatments were evaluated 30 days after inoculation by counting the number of affected spikelets per spike. Results were analyzed using categorical data analysis.

Supplementary moisture in the form of mist irrigation was provided on the four rainless days.  The different treatments were evaluated 30 days after inoculation by counting the number of affected spikelets per spike.  Results were analyzed using categorical data analysis.

Results and Discussion

Results of the analysis of variance (Table 1) showed highly significant differences at 0.001% between isolates, cultivar, and cultivar x isolate interactions. Isolate 1 from El Tigre was the most virulent, and 7 and 8 from Patzcuaro the least virulent (Table 2). The cultivar Frontana showed the best resistance to the eight isolates used (Table 3). Table 4 shows the absolute ratio of infected: healthy grains for each wheat cultivar with every isolate.

Table 1. Analysis of variance of blighted spikelets of five wheat cultivars inoculated with eight individual Fusarium graminearum isolates, Atizapan, Toluca, Mexico, 2000.

 Table 2. Analysis of contrast among the five test wheat cultivars.
Cultivars 2 3 4 5
Source DF Chi Square Prob 1. *** *** *** NS
Cultivar 4 565.02 *** 2. * *** ***
Isolate 7 178.10 *** 3. *** ***
Cultivar x isolate 28 106.38 *** 4. ***

 

Table 3. Analysis of contrast among the eight Fusarium graminearum isolates Table 4. Absolute ratios of infected: healthy grains on four resistant and one susceptible wheat cultivars inoculated with eight different Fusarium graminearum isolates from four regions of Mexico. Atizapan, Toluca, Mexico, 2000.
Isolates 2 3 4 5 6 7 8
1 *** *** *** *** *** *** ***

Isolates
2 ** NS NS NS NS NS Cultivars 1 2 3 4 5 6 7 8
3 * ** ** ** ** Sha/4Chil 0.194 0.098 0.096 0.103 0.093 0.135 0.085 0.090
4 NS NS * * Catbird 0.111 0.060 0.880 0.880 0.054 0.046 0.053 0.062
5 NS NS NS Sumai#3 0.090 0.046 0.079 0.055 0.053 0.065 0.059 0.054
6 NS NS Frontana 0.055 0.051 0.058 0.035 0.039 0.051 0.039 0.043
7 NS Flycatcher 0.210 0.127 0.131 0.132 0.131 0.084 0.108 0.082

 

Conclusion

These results confirm the genotype x isolate interactions observed in Toluca in 1998. The data are unique because no other study has detected this interaction. These results help to understand the differential reaction (susceptible-resistant) observed in some varieties in different locations. The reason these results have not been replicated is that the appropriate genotypes and isolates have not been used, that is, it is necessary to choose those individuals within the host and pathogen populations that would allow detection of this event.

 

References

Gilchrist, L., S. Rajaram, M. van Ginkel, M. Kazi, and J. Franco. 1997. Characterizing Fusarium graminearum resistance of CIMMYT bread wheat germplasm. Fifth European Fusarium Seminar. Szeged, Hungary.

Mesterhazy, A. 1997. Breeding for resistance to Fusarium head blight of wheat. In: Fusarium Head Blight: Global Status and Future Prospects. H.J. Dubin, L. Gilchrist, J. Reeves, and A. McNab, eds. Mexico, D.F.: CIMMYT.


© CIMMYT April 2001

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