Susceptibility of leguminous green manure species to Rhizoctonia solani and Sclerotium rolfsii

We studied the susceptibility of species used as green manure in common bean fields to root rot (Rhizoctonia solani) and southern blight (Sclerotium rolfsii). Seeds of Crotalaria breviflora, Canavalia ensiformis, Cajanus cajan , Dolichos lablab , Stizolobium cinereum , S. aterrimum, and the bean cvs. “Pérola”, “Valente” and “Carnaval” were sown in soil infested by either R. so lani AG-4 or S. rolfsii in greenhouse. The emergence of D. lablab seedlings in soil infested by Paula Júnior, T.J.; Teixeira, H.; Vieira, R.F.; Lehner, M.S.; Lima, R.C.; Queiroz, T.F.N. Susceptibility of leguminous green manure species to Rhizoctonia solani and Sclerotium rolfsii. Summa Phytopathologica, v.37, n.4, p.218-220, 2011. Palavras-chave adicionais: Phaseolus vulgaris, crop rotation, soilborne pathogen, integrated management. R. solani dropped to 62%. C. breviflora, C. ensiformis and cv. “Valente” presented the lowest root rot severity. The pathogen S . rolfs ii drastically reduced seedling emergence in all species; no C. cajan and S . cinereum seedling emerged. All plant species presented high southern blight severity. We conclude that leguminous crops are not suitable as green manure for areas of bean cultivation with high R. solani and S. rolfsii populations.


RESUMO
so lan i foi reduzida a 62 %. C. b rev iflora, C. en sifo rmis e Valente a presenta ra m a menor severidade de podridã o-ra dicu lar. O fu ngo S. rolfsii reduziu drasticamente a emergência de toda s as espécies; nenhu ma plâ ntu la de C. c a ja n e S . c in e re u m emergiu . Toda s a s espé ci es a p re se nt a r a m a l ta seve ri da de d e p od ri dã o-d o-co lo . As espécies de legu minosa s testa da s nã o são apropriada s para serem utilizadas como adubo verde em áreas de cultivo de feijão com alta população de R. so lan i e S. rolfsii. ]. R. solani is the cause of many diseases in a broad array of host plant species (1). Most isolates of R. solani recovered from roots of common beans are members of the anastomosis group AG-4, but isolates of AG-2 have also been reported (7). Seed and seedling infections result in severe pre-and postemergence damping-off or root rots, which reduce plant stands (3). In the absence of host tissue, the pathogen survives in the soil as sclerotia and melanized mycelium, free or embedded in soil organic debris, originating from parasitized crops and weed hosts and/or from saprophytically colonized crop residues, comprising sources of inoculum for the following crop (5). The fungus S. rolfsii is one of the most widely distributed and destructive pathogens of several economic crops (1). In common beans, the disease caused by S. rolfsii is called southern blight. Infections of beans occur at the soil surface or just bellow it, at any stage of plant growth. Symptoms include damping-off, lesions on lower stem and root tissues, progressive yellowing, wilting, and premature defoliation (1). Control efforts have often met with limited success, due in part to the extensive host range, prolific growth, and ability to produce large numbers of sclerotia that may persist in the soil for at least one year (4).
Integrated strategies including crop rotation may be used to provide a satisfactory control of R. solani and S. rolfsii in common beans. Species of Leguminosae used as green manure crops can fix nitrogen (N) from atmospheric air, enriching the soil with N. They also add many tons of organic matter to the soil, thereby improving water-holding capacity, nutrient content, friability, and texture of the soil. Furthermore, some species are used to control nematodes.
However, the role of these plants as hosts of pathogens inhabitants of soil that generally infect dry beans is not clear yet. Crop rotation with legumes may have serious epidemiological implications for common beans, since infected plants increase the inoculum in the soil. We investigated the susceptibility of legumes to R. solani and S. rolfsii and the risks of the recommendation of crop rotation including legumes in fields infested by both pathogens.
Crotalaria breviflora DC., Canavalia ensiformis (L.) DC., Cajanus cajan (L.) Millspaugh, Dolichos lablab L., Stizolobium cinereum Piper & Tracy, Stizolobium aterrimum Piper & Tracy, and the bean cvs. "Pérola", "Valente" and "Carnaval" were tested for their susceptibility to R. solani (AG-4) or S. rolfsii. The pathogens were isolated from infected dry bean plants and cultivated in potato-dextrose-agar medium for 10 days at 25 °C. Two 5 mm diameter mycelial-agar disks were transferred from the margin of growing colonies to 200 mL-Erlenmeyer flasks with 50 g of autoclaved rice grains. After six days of incubation in darkness at 25 °C, rice grains were dried on trays for 24 h before being used. The inoculum of each fungus was mixed with soil (2 g kg -1 of soil) and put in 1 kg pots. The soil (red-yellow podzolic Cambic; pH = 5.9; Ca = 1.7 cmol c dm -3 ; Mg = 0.8 cmol c dm -3 ; Al = 0.0 cmol c dm -3 ; H+Al = 2.5 cmol c dm -3 ; P = 9.0 g dm -3 ; K = 42.0 g dm -3 ; and organic matter = 22.0 g kg -1 ) was previously solarized for eight weeks using a transparent polyethylene film (1.5 mm). In a non-infested control, non-colonized autoclaved rice grains were mixed with solarized soil. Each pot was sown with three seeds of each legume at 3 cm depth. Pots were maintained in greenhouse and irrigated once a day with the same amount of water. A randomized complete block design with five replicates was used. The number of emerged seedlings was counted at 10 days after sowing (DAS). Hypocotyls were evaluated at 30 DAS for disease severity according to the 1-9 scale proposed by Abawi & Pastor-Corrales (1). The disease severity index (DSI) was calculated: DSI(%) =  (scores of all plants)/[9 x (total number of plants)] x 100.
The emergence of seedlings in the non-infested control soil was 100%. The seedlings emerged from soil infested by S. rolfsii were fewer and the severity was higher compared with those from soil infested by R. solani (Fig. 1). The emergence of D. lablab in soil infested by R. solani dropped to 62% (Fig. 1A). C. breviflora, C. ensiformis and "Valente" presented the lowest severity of root rot (Fig. 1A). S. rolfsii reduced the emergence of all species; no C. cajan and S. cinereum seedling emerged (Fig. 1B). All species presented high severity of southern blight (Fig. 1B). R. solani or S. rolfsii were re-isolated from all plants with symptoms of root rot or southern blight. We concluded that the use of C. breviflora, C. ensiformis, C. cajan, D. lablab, S. cinerium and S. aterrimum as green manure crops in bean fields can contribute to increase or at least to maintain the inoculum of R. solani and S. rolfsii in the soil. Therefore, these species are not suitable as green manure for fields with high population of the pathogens. Berni et al.
(2) and Toledo-Souza et al. (6) also found that legume crop residues can increase soil populations of Rhizoctonia spp. and recommended that legumes should be avoided as previous crops to common beans. We suggest fallow or sowing of Gramineae to reduce the inoculum of the pathogens in infested areas, since it has been demonstrated that rotations of non-host crops may be effective in controlling R. solani (6) and S. rolfsii (4). Our results can be useful for the establishment of management programs of fields infested by R. solani and S. rolfsii.

ACKNOWLEDGMENTS
Research supported by FAPEMIG and CNPq.