Fusarium stalk rot is a severe disease of maize causing pre-harvest stalk breakage, which, if it occurs early, reduces grain yield and makes harvesting much more difficult. Additionally, the crop is contaminated by mycotoxins that are harmful to humans and animals. A cost-effective and environmentally friendly way of reducing the damage caused by stem rot is to breed resistant varieties. However, resistance is controlled by many genes and phenotypically shows a quantitative distribution from high to low resistance. Therefore, artificial infection and reliable screening of genotypes are essential for the successful selection of more resistant maize varieties. Our study aimed to identify the best methods to accurately screen genotypes for stalk rot resistance in breeding programmes in North-western Europe. The main requirements for use in practical breeding are methods that are easy to use, have high throughput and are cost-effective for screening large populations.
In our study, we tested the resistance of 22 maize hybrids to Fusarium stalk rot using four inoculation methods at two locations each over two years. The inoculation methods included needle injection of fungal spores directly into the stem, a method in which Fusarium-colonised toothpicks were inserted into the stem, a stick method in which a stick with a rough nail was moistened with the liquid inoculum and inserted into the stem, and a mycelial method in which Fusarium-colonised wheat grains were placed around the stem of the maize plants. All inoculations were made during maize flowering and all methods were carried out with three Fusarium species known to be common in European maize: F. graminearum, F. culmorum and F. temperatum. Needle injection showed the highest stalk rot severity in maize hybrids, followed by the toothpick and stick methods. The mycelium method gave too low an infection to be used. At harvest, we split the maize stems by hand and measured the visible infected tissue that was either necrotic and/or covered by mycelium (=full length of infection).
The results showed that in principle each method was useful. However, they differed in the severity of stalk rot. Needle injection was the best, only F. temperatum had slightly higher severity with the toothpick method. Stick method always resulted in the lowest severity. F. graminearum was the most aggressive fungal pathogen, causing the highest stalk rot severity with each method. Needle injection is now being used to screen large numbers of genotypes to select the most resistant ones.
Comparison of four inoculation methods and three Fusarium species for phenotyping stalk rot resistance among 22 maize hybrids (Zea mays)
Figure: Measuring length of infection on artificially infected maize stalks (left) and results of the experiment with three methods and three Fusarium species (right)
