Yunyue Wang

Genetic diversity and disease control in rice

Crop heterogeneity is a possible solution to the vulnerability of monocultured crops to disease. Both theory and observation indicate that genetic heterogeneity provides greater disease suppression when used over large areas, though experimental data are lacking. Here we report a unique cooperation among farmers, researchers and extension personnel in Yunnan Province, China—genetically diversified rice crops were planted in all the rice fields in five townships in 1998 and ten townships in 1999. Control plots of monocultured crops allowed us to calculate the effect of diversity on the severity of rice blast, the major disease of rice. Disease-susceptible rice varieties planted in mixtures with resistant varieties had 89% greater yield and blast was 94% less severe than when they were grown in monoculture. The experiment was so successful that fungicidal sprays were no longer applied by the end of the two-year programme. Our results support the view that intraspecific crop diversification provides an ecological approach to disease control that can be highly effective over a large area and contribute to the sustainability of crop production.

Panicle Blast and Canopy Moisture in Rice Cultivar Mixtures

ABSTRACT Glutinous rice cultivars were sown after every fourth row of a nonglutinous, hybrid cultivar in an additive design. The glutinous cultivars were 35 to 40 cm taller and substantially more susceptible to blast than was the nonglutinous cultivar. Interplanting of glutinous and nonglutinous rice reduced the incidence and severity of panicle blast on the glutinous cultivars by >90%, and on the nonglutinous cultivar by 30 to 40%. Mixing increased the per unit area yield of glutinous rice by 80 to 90% relative to pure stand, whereas yield of the nonglutinous cultivar was essentially unaffected by mixing. To determine whether the different plant heights and canopy structures may contribute to a microclimate that is less favorable to blast infection, we monitored the moisture status of the glutinous cultivars in pure stand and mixture at 0800 h by measuring relative humidity at the height of the glutinous panicles using a swing psychrometer and by visually estimating the percentage of leaf area covered by dew. Averaged over the two seasons, the number of days of 100% humidity at 0800 h was 20.0 and 2.2 for pure stands and mixtures, respectively. The mean percentage of glutinous leaf area covered by dewwas 84 and 36% for the pure stands and mixtures, respectively. Although other mechanisms also were operative, reduced leaf wetness was likely a substantial contributor to panicle blast control in the mixtures.

Crop diversity for yield increase.

Traditional farming practices suggest that cultivation of a mixture of crop species in the same field through temporal and spatial management may be advantageous in boosting yields and preventing disease, but evidence from large-scale field testing is limited. Increasing crop diversity through intercropping addresses the problem of increasing land utilization and crop productivity. In collaboration with farmers and extension personnel, we tested intercropping of tobacco, maize, sugarcane, potato, wheat and broad bean – either by relay cropping or by mixing crop species based on differences in their heights, and practiced these patterns on 15,302 hectares in ten counties in Yunnan Province, China. The results of observation plots within these areas showed that some combinations increased crop yields for the same season between 33.2 and 84.7% and reached a land equivalent ratio (LER) of between 1.31 and 1.84. This approach can be easily applied in developing countries, which is crucial in face of dwindling arable land and increasing food demand.

The HD-GYP Domain Protein RpfG of Xanthomonas oryzae pv. oryzicola Regulates Synthesis of Extracellular Polysaccharides that Contribute to Biofilm Formation and Virulence on Rice

Bacterial leaf streak caused by Xanthomonas oryzae pv. oryzicola (Xoc) is one of the most important diseases in rice. However, little is known about the pathogenicity mechanisms of Xoc. Here we have investigated the function of three HD-GYP domain regulatory proteins in biofilm formation, the synthesis of virulence factors and virulence of Xoc. Deletion of rpfG resulted in altered production of extracellular polysaccharides (EPS), abolished virulence on rice and enhanced biofilm formation, but had little effect on the secretion of proteases and motility. In contrast, mutational analysis showed that the other two HD-GYP domain proteins had no effect on virulence factor synthesis and tested phenotypes. Mutation of rpfG led to up-regulation of the type III secretion system and altered expression of three putative glycosyltransferase genes gumD, pgaC and xagB, which are part of operons directing the synthesis of different extracellular polysaccharides. The pgaABCD and xagABCD operons were greatly up-regulated in the Xoc ΔrpfG mutant, whereas the expression of the gum genes was unaltered or slightly enhanced. The elevated biofilm formation of the Xoc ΔrpfG mutant was dramatically reduced upon deletion of gumD, xagA and xagB, but not when pgaA and pgaC were deleted. Interestingly, only the ΔgumD mutant, among these single gene mutants, exhibits multiple phenotype alterations including reduced biofilm and EPS production and attenuated virulence on rice. These data indicate that RpfG is a global regulator that controls biofilm formation, EPS production and bacterial virulence in Xoc and that both gumD- and xagB-dependent EPS contribute to biofilm formation under different conditions.

Intercropping of rice varieties increases the efficiency of blast control through reduced disease occurrence and variability

Abstract Creating a crop-heterogeneous system by intraspecific mixtures of different rice varieties can substantially reduce blast diseases. Such variety mixtures provide an ecological approach for effective disease control, maintaining high yields with the minimum fungicide applications. Whether such an approach is universally applicable for random rice variety combinations and what is the variation pattern of the diseases under intercropping still remains unclear. We conducted two-year large-scale field experiments involving 47 rice varieties/lines and 98 variety-combinations to compare the occurrence of rice blast in monoculture and intercropping plots at multiple sites. In the experiments, the plant height of the selected traditional varieties was about 30 cm taller, and their life cycle was 10 days longer, than that of the improved rice varieties. The monoculture included either traditional or modern rice varieties grown in separate plots. The intercropping included both traditional and modern rice varieties planted together in the same plots. Results from the field experiments under natural disease conditions demonstrated significant reduction for rice blast disease in intercropping plots, compared with that in monoculture plots. For traditional varieties, the average blast incidence reduced from ~26% in monoculture to ~10% in intercropping, and the disease severity reduced from ~17 in monoculture to ~5 in intercropping. For modern varieties, the average blast incidence reduced from ~19% in monoculture to ~10% in intercropping, and the severity from ~10 in monoculture to ~4 in intercropping. Traditional rice varieties (~72%) had a much greater increase in the efficiency of disease control than modern varieties (~60%). In addition, substantially lower values of variance in the blast incidence and severity was detected among the variety combinations in intercropping plots than in monoculture plots. Based on these results, we conclude that the intercropping or mixture of rice varieties greatly reduces the occurrence and variation of rice blast disease in particular variety combinations, which makes the intercropping system more stable and consistent for disease suppression on a large scale of rice cultivation.