Phytonematodes infesting common bean fields in Brazil, and pathogenicity tests with Pratylenchus brachyurus

Abstract

Phytonematodes are among the main pathogens of the common bean. In general, control practices that aim to control these pathogens have not been regularly adopted in Brazil. The objectives of the present study were to evaluate the occurrence of phytonematodes in dry bean fields in Paraná and São Paulo states and estimate the pathogenicity of Pratylenchus brachyurus to dry bean in glasshouse experiments. Root and soil were sampled for nematode extraction, identification and estimation of population density. Four glasshouse experiments with different initial population densities were carried out to evaluate the effect of P. brachyurus on the growth of dry bean plants. Six species of phytonematodes were detected, namely Helicotylenchus dihystera , P. brachyurus , Pratylenchus crenatus , Meloidogyne javanica , Meloidogyne incognita and Rotylenchulus sp. The spiral nematode H. dihystera was found in all samples and was the most abundant species in both states. The lesion nematode P. brachyurus was also frequent, reaching 94% in Paraná and 100% in São Paulo. The root-knot nematodes and Rotylenchulus sp. were reported only in São Paulo fields (45% and 18% frequency, respectively), and P. crenatus only in Paraná (12%). The most abundant nematode in the soil was H. dihystera , and in roots was P. crenatus. It was demonstrated that densities of 6.66 specimens of P. brachyurus per cm 3 of soil provoke visible and measurable root decay. In conclusion, survey data showed low phytonematode densities and pathogenicity tests demonstrated that densities above 6.67 specimens of P. brachyurus per cm 3 of soil cause damage to the dry bean roots. and forty composite samples in (September 2011 in corresponding to late winter in the Southern Hemisphere). An additional survey was conducted from November 2012 to January 2013 (corresponding to mid-spring until mid-summer, in the Southern Hemisphere) in two municipalities of São Paulo state (Itaí and Itapeva), in which 13 composite samples were collected, exclusively to obtain additional specimens of Meloidogyne spp. A composite sample was taken from each field of variable sizes (1 to 86 ha), consisting of ten randomly collected subsamples of roots and soil. Each root subsample was composed of roots from ten (up to 1-month-old) or five (at least 2-months-old) bean plants. All plants were collected at the vegetative phase (V4) or the beginning of the reproductive phase (R6). Soil from each sampled plant’s rhizosphere (0–20 cm depth) constituted a soil subsample. Each of the composite samples of roots and soil was placed separately in plastic bags. In the laboratory, the root system was cut into 2-cm-length pieces and mixed thoroughly. Then, a subsample of 10 g was taken for nematode extraction by blender centrifugal-flotation method (COOLEN; D´HERDE, 1972) with kaolin and sucrose solution (d = 1.15 g∙mL –1 ), using a centrifuge with four 138-cm 3 -tubes (4 cm diameter and 11 cm height) at 1,800 rpm and 580 g. Similarly, the soil was mixed systematically, and a subsample of 250 cm 3 was taken and processed by the centrifugation-flotation method 1964). tests demonstrated that densities above 6.67 specimens of P. brachyurus per cm3 soil cause damage to the dry bean roots. Survey data showed low phytonematodes densities. The low or undetectable crop losses reported by dry bean growers in sampled areas of the state of Paraná and São Paulo might be attributed to the low phytonematode densities here presented. However, growers should adopt production systems to prevent an increase in nematode population, especially of P. brachyurus , M. incognita and M. javanica , which are still low in numbers but could lead to future yield losses, these nematode species to be employed.