Kenta Uesugi

Effect of Introducing Nematode-Resistant Sweet Potato Cultivars on Crop Productivity and Nematode Density in Sweet Potato-Radish Double-Cropping Systems

Abstract The root-knot nematode (RKN) is a significant pest in upland farming. We studied the effects of introducing nematode-resistant sweet potato cultivars on crop yield, crop quality, and RKN population dynamics in sweet potato-radish double-cropping systems. Three cropping systems with and without nematicide treatment (6 systems in total) were arranged for a 4-yr field experiment from 2003 to 2006. In two nematode-suppressive cropping systems, highly nematode-resistant J-red, (J) or Sunny red (S) and moderately nematode-resistant Kyushu No. 139 (K139) or Murasakimasari (M) sweet potato cultivars were cropped in alternate years beginning with the former and the latter, and in the non-nematode-suppressive cropping system, nematode-susceptible Kokei No. 14 (K14) and M were cropped in alternate years beginning with the former, from 2003 to 2005. In all cropping systems, K14 was cropped in 2006 to estimate the nematode-suppressive effect of the preceding 3-yr cropping. Introduction of J and S to the cropping system decreased the number of RKNs. In 2006, the extent of injury of K14 was decreased in nematode-suppressive cropping systems. The RKN population density, however, recovered during the cropping of K14 even after cropping of J or S or after nematicide treatment. This suggests that the effects of these measures last for only 1 yr. Nematode injury in radish decreased after nematicide treatment and after cropping of highly nematode-resistant J or S. These results indicate that the introduction of nematode-resistant sweet potato cultivars in cropping systems is effective for reduction of agrochemical use for sustainable agriculture.

Suppressive effects of palisade grass (Brachiaria brizantha) on Pratylenchus coffeae

The suppressive effects of palisade grass (Brachiaria brizantha) cv. MG5 on Pratylenchus coffeae were tested in pot and seedling tray experiments and under field conditions. In pot tests, four isolates of P. coffeae were tested by inoculating 500 mixed life stages of each isolate on 5- or 7-week-old palisade grass plants. The nematode number in palisade grass was significantly lower than in susceptible cucumber in all isolates and below 20% of the inoculated level 69-86 days after inoculation. In seedling tray tests, the number of eggs and adults + juveniles in palisade grass roots was examined 5-25 days after inoculation (dai) of 200 mixed life stages of an isolate of P. coffeae to a 3-week-old seedling. The number of adults + juveniles in palisade grass roots was not statistically different from sorghum and black oat 5 and 10 dai, while the number of eggs and adults + juveniles was significantly lower than these two plants 20 and 25 dai. In field plot tests, palisade grass was cultivated for approximately 4 months in 2011 and 2012. The P. coffeae density after palisade grass cultivation was significantly lower than that after bare fallow in 2011, whereas the nematode density was not significantly different between these two treatments in 2012. Rates of lesions on sweet potatoes cultivated after growing palisade grass were significantly lower than those after growing sorghum or guinea grass, but not significantly different from that after fallow. These results indicated that the cultivation of palisade grass cv. MG5 has a nematode suppressive effect that is comparable to fallow treatments.

Suppressive effect of black oat, Avena strigosa, KH1a on Meloidogyne spp.

The suppressive effect of a black oat, Avena strigosa , breeding line KH1a on Meloidogyne spp. was examined in pot tests and on Meloidogyne incognita in pot and field tests. In pot tests, roots of black oats were examined 42-46 days after inoculation of 500 second-stage juveniles. There were significantly fewer egg masses on the roots of KH1a than on 12 black oat cultivars examined. KH1a was a poor host for four isolates of M. incognita , two isolates of M. arenaria and one isolate of M. javanica , and a non-host for one isolate of M. hapla . The effect of autumn and spring cropping of KH1a on soil nematode density was examined in M. incognita -infested fields. Nematode density after autumn cropping of KH1a was significantly lower than that after susceptible black oat, resulting in significantly lower P f / P i in KH1a (0.10 and 0.13) than in susceptible black oat (0.42 and 0.74). Damage indices of the succeeding crop, sweet potato, were significantly lower in KH1a plots than in susceptible black oat plots. In spring cropping, there were significantly fewer soil nematodes in KH1a plots than in susceptible black oat plots 3 weeks after cultivation. In both autumn and spring cropping, there was no significant difference in soil nematode density between KH1a and bare fallow. These results suggested that KH1a is a good alternative to current susceptible cultivars for the management of M. incognita .