Occurence, disease cycle, effects on yield, and management of root knot disease of onion (Allium cepa L.) caused by Meloidogyne graminicola Golden and Birchfield

Results of the study showed that two days after inoculation (DAI), the second-stage juveniles (J2) were in the meristematic region of the root tips. Root galls became apparent 5 DAI. J3 and J4 were first observed 5-6 DAI, young females and 13 DAI, and mature female with eggs at 17 DAI. Nematode life cycle from J2 to eggs took 17 days at 25-32 deg C. Root-knot infected plants have short galled roots, smaller bulbs than normal, or no bulbs developed. The pathogen infected 30 onion cultivars, 153 field crops and 85 weed species. All onions were severely galled regardless of type of cultivar. Wheat, soybean, garden pea, mustard, winged bean, pechay, Chinese cabbage, lettuce, and snap beans were good hosts, but pepper, cucumber, mungbean, corn, peanut, sitao, cowpea, and members of Cucurbitaceae and Solanaceae (except tomato) were poor or non-hosts. Many weeds species from rice-onion fields, including members of Cyperaceae, served as alternate hosts. Initial nematode population (Pi) and age of transplant influenced growth and yield of yellow Granex onion. Onion yield, root and leaf weights decreased with increasing age of transplant. At vegetative stage, leaf weight and root length decreased with increased Pi but low in Pi mildly stimulated plants height. Growth and yield at harvest decreased with increased Pi. Bulb weight was reduced by 7-82% and diameter by 10-62% when plants were inoculated with 50-10,000 J2. Bulb weight of Red Creole was reduced by 86% and `Batanes' by 60%. Bulblets of `Tanduyong' were reduced by 31% and 48% in weight and number, respectively. Onion bulbs in the field were reduced by 16-35% in weight and 60-18% in diameter, when roots were 10-100% galled. Crop rotation with continuous three croppings of non-hosts crops between rice and onion reduced soil Pi. Soil amendment with organic fertilizer or composts had no effect on onion yield and M. graminicola but Mykovam inoculant improved seedling growth. Farmer's standard plowing before cropping reduced soil Pi but did not affect onion yield. Rice hull burning (RHB) reduced soil Pi and increased onion yield. Increasing the thickness of rice hulls from 15 cm deep to 30 cm deep increased yield by 44% over no RHB and 12% over 15-cm deep rice hulls and produced 38% more large bulbs for export over 15-cm deep burned rice hulls, and 152% over no RHB. Soil organic matter, phosphorus (P), and exchangeable potassium (K) increased in plots with burned rice hulls. Formulated products of Bacillus sp. with code names LEP 118 at 11%, APP 48 at 25%, and PG14 at 11% or 25% reduced J2 and galled roots in Yellow Granex 3 DAI. Bulb weight and diameter of Yellow Granex inoculated with a mixture of Glomus mosseae, G. fasciculatum, and Mykovam at sowing increased by 54% and 24% in nematode-free soil and by 262% and 96%, respectively, in M. graminicola-infested soil. Bulb weight and diameter of Red Creole were increased by 23% and 13%, respectively, only with G. mosseae and where M. graminicola was not present. VAM fungi enhanced P uptake of the plants and increased plant's tolerance to M. graminicola. Among the strategies, only RHB contributed greatly in onion yield increase and also decreased nematode and weed populations. The effect of smoke, however, is an environmental concern. Integration of different approaches, such as crop rotation with non-host crops, good land preparation, application of VAM fungi at seedbed to produce vigorous seedlings, and root-dip treatment with Bacillus sp. at transplanting is an important emerging alternative to manage root-knot disease in rice-onion system to reduce economic losses without negative effects on human health and the environment