Full Length Research Paper
ABSTRACT
Soybean cyst nematode, Heterodera glycines Ichinohe, is one of the major phytopathological problems affecting soybeans, Glycine max (L.) Merr., in the major producing countries and the use of resistant cultivars and crop rotation have been the main methods of control adopted to reduce the nematode population in infested soils. Purpose of this study was to evaluate the effect of resistant (BRSGO Ipameri and BRSGO Chapadões) and susceptible (BRSGO Araçu, BRSGO Jataí, BRSGO Luziânia, BRS Favorita RR, BRS Valiosa RR, BRS Silvânia RR) soybean cultivars on the development of H. glycines males and females during two successive years (2007 and 2008). In the trial of 2008 the plants were divided in three plots, with the last one having the roots stained to count the juveniles and to evaluate survival rate. Resistant cultivars always maintained a small number of females and males, except for cultivar BRSGO Ipameri that had a high count of males. Only cultivars BRS Favorita RR and BRS Silvânia RR had a sex ratio of 1:1. All other susceptible cultivars had, in general, greater number of males than females. Survival rate was nil on both resistant cultivars, and varied from 6.75 to 35.00% on the susceptible cultivars.
Key words: Glycine max, cyst nematode, sex ratio, hydroponics.
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
Significant differences for the number of H. glycines females in the roots among the cultivars were observed for both experiments, in 2007 and 2008 (Tables 1 and 2). The commercial cultivars resistant to H. glycines, race 14, BRSGO Ipameri and BRSGO Chapadões, confirmed the expected performance in both experiments, presenting smaller numbers of females (Tables 1 and 2). All other cultivars performed as susceptible to the nematode.
In 2007, the number of eggs per female on the cultivar BRSGO Ipameri differed statistically from the other cultivars presenting the lowest number. The cultivar BRSGO Chapadões had low number of eggs per female but did not differ statistically from BRS Araçu and BRS Valiosa RR. In 2008 both resistant cultivars, BRSGO Ipameri and BRSGO Chapadoes, had low development of eggs per female differing from the susceptible cultivars (Tables 1 and 2).
A high number of H. glycines males were found in the roots of plants from the hydroponic system in both experiments (Tables 1 and 2). In 2007, the cultivar BRSGO Chapadões differed from all the other cultivars presenting the lowest number of males. Cultivar BRSGO Ipameri only differed from BRS Valiosa RR, BRSGO Luziânia and BRSGO Jataí (Table 1). In 2008 the cultivar BRSGO Chapadões presented the lowest number of males differing from cultivars BRS Favorita RR, BRS Valiosa RR and BRS Silvânia RR (Table 2).
The sex ratio between males and females in 2007 (Table 1), is near 50% only for cultivars BRS Favorita RR and BRS Silvânia RR, by the χ² test (5%). In 2008 all cultivars had the sex ratio different from 1:1. The resistant cultivars had sex ratios extremely high due to the absence or very low development of females. The survival rate, done only for the 2008 experiment, varied from 0% to 35.00 % (Table 2). All root systems were colonized by the nematode however, females developed only in the root system of susceptible cultivars.
The greater number of females found in 2007 than in 2008 may be explained by the greater initial inoculum concentration in the first experiment and, also, by the inoculum condition, which had been collected from the field, presenting greater virulence in 2007 than the population maintained in the greenhouse for several generations until the next year. Brito et al. (1999) and Koenning (2000) also observed that the increase in the initial concentration of inoculum of H. glycines, tends to increase the number of females present in the root of soybean system.
Genetic resistance to the soybean cyst nematode affects the number of male individuals, as highlighted by the evaluations of these trials. Cultivar BRSGO Chapadões, presented the smallest number of males in both experiments. However, cultivar BRSGO Ipameri, which is also resistant to H. glycines, race 14, had greater number of males than BRSGO Chapadões. This difference in values is probably due to the source of resistance of each of these cultivars. Cultivar BRSGO Chapadões had PI 437654 as genetic background, while cultivar BRSGO Ipameri had PI 88788 (Dias et al., 2007). Studies done by Luedders (1987) and Colgrove and Niblack (2005) demonstrated that greater proportions of males are found in PI 88788 than in PI 437654, as a function of differential mortality of males and females.
Halbrendt et al. (1992) also found effect of the source of resistance on the development of H. glycines male individuals which confirms the results found in this study for both resistant cultivars. These authors confirmed that the resistance that affects the development of J3 and J4, as the resistance from PI 209332, provides higher male development than the sources of resistance that affects the stages J2 and J3, like the resistance from cultivar Pickett. This occurs due to the feeding period of the males. H. glycines males only feed during the stages J2 and J3 while females feed from J2 until adult (Endo, 1964, 1965). Therefore, the sources of resistance that are not effective during the early stages of development result in lower mortality of males.
It was expected to find a sex ratio of 1:1 in all susceptible soybean cultivars, since, according to Luedders (1987), Halbrendt et al. (1992) and Colgrove and Niblack (2005), resistance is one of the stress factors that inhibits H. glycines female development and thus causes a differential death among male and female. In general, the number of males found, especially in the resistant cultivars, was greater than the number of females. Koliopanos and Triantaphylou (1972) found that under greater population densities, a trend of forming more male than female individuals existed, especially with the inoculation of 5,000 eggs and J2. Considering that the initial population in the substrate used was greater than 5,000 eggs and J2, it may explain the greater proportion of H. glycines males found.Survival rate was 0% for both resistant cultivars, which is near the values found by Colgrove and Niblack (2005) for the plant introductions PI 88788 and PI 437654. In contrast, the susceptible cultivars had survival rates varying from 6.75 to 35.00%. These values corroborate those found by Evans and Fox (1977) and Colgrove and Niblack (2005) for the soybean cultivar used as susceptibility standard, Lee. Colgrove and Niblack (2005) found survival rates varying from 41.00 to 112.00% for several resistance sources evaluated in their experiments. Halbrendt et al. (1992) reported mortality rate varying from 23.00 to 51.00% and stated that high mortality percentage is normal among juveniles.
The developments of H. glycines males do not get the same proportion as the development of females in the same soybean cultivar. The results presented in this study suggest that the differential development of males and females occur as a function of resistance sources of soybean cultivars. The Brazilian resistant cultivars BRSGO Ipameri and BRSGO Chapadoes are effective on controlling H. glycines by the reduction of female development although allowing high male development.
CONFLICT OF INTEREST
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