Identification of combined resistance to Meloidogyne enterolobii and M. incognita in sweetpotato genotypes.

Root-knot nematodes, genus Meloidogyne, are the most damaging nematodes of sweetpotato causing yield reduction and aesthetic damage of the marketable product. Several sweetpotato cultivars currently grown in the United States have intermediate to high resistance to Meloidogyne incognita, however, many of these cultivars are susceptible to M. enterolobii. Therefore, the response of 69 sweetpotato genotypes to M. enterolobii and M. incognita was evaluated under greenhouse conditions to identify potential sources of resistance. The cultivars 'Beauregard' and 'Jewel' were used as controls. Results showed that sweetpotato genotypes were either highly resistant or highly susceptible to M. enterolobii, while they showed a wide spectrum response to M. incognita ranging from highly susceptible to highly resistant. Twenty-six genotypes were resistant to M. enterolobii and eleven genotypes were resistant to M. incognita. Combined resistance to M. enterolobii and M. incognita was observed in three sweetpotato genotypes. Selected genotypes from this study will be used to incorporate the observed resistance into a commercially viable sweetpotato cultivar.

The AVR4 effector is involved in cercosporin biosynthesis and likely affects the virulence of Cercospora cf. flagellaris on soybean.

One of the most devastating fungal diseases of soybean in the southern USA is Cercospora leaf blight (CLB), which is caused mainly by Cercospora cf. flagellaris. Recent studies found that the fungal effector AVR4, originally identified in Cladosporium fulvum as a chitin-binding protein, is highly conserved among other Cercospora species. We wanted to determine whether it is present in C. cf. flagellaris and, if so, whether it plays a role in the pathogen infection of soybean. We cloned the Avr4 gene and created C. cf. flagellaris ∆avr4 mutants, which produced little cercosporin and significantly reduced expression of cercosporin biosynthesis genes. The ∆avr4 mutants were also more sensitive to chitinase and showed reduced virulence on soybean compared to the wild-type. The observed reduced virulence of C. cf. flagellaris ∆avr4 mutants on detached soybean leaves is likely due to reduced cercosporin biosynthesis. The phenotypes of reduced cercosporin production and cercosporin pathway gene expression, similar to those of the ∆avr4 mutants, were reproduced when wild-type C. cf. flagellaris was treated with double-stranded RNA targeting Avr4 in vitro. These two independent approaches demonstrated for the first time the direct involvement of AVR4 in the biosynthesis of cercosporin.