EFFECT OF INOCULUM LEVEL, TYPE, PLANT AGE AND ASSESSMENT DATE ON EVALUATING SUGAR BEET RESISTANCE METHODS FOR ROOT-KNOT NEMATODE, Meloidogyne incognita

Abstract

The objectives of this study were to determine the effect of inoculum level, inoculation date, assessment date and inoculum type, on evaluating M. incognita resistance in sugar beet, and to optimize the resistance screening technique used to categorize root-knot nematode resistant sugar beet cultivars under the greenhouse conditions (25±2.5°C). A series of greenhouse tests were done using seven sugar beet varieties with three levels of resistance to M. incognita. The three resistance levels could be separated based on gall indices as early as two weeks after inoculation (WAI) using 6000 eggs of M. incognita per plant. Results indicated that based on gall index, low inoculum level (500 and 1000 eggs/ plant) could separate four sugar beet varieties from each other only on the fourth assessment date (8 WAI) for inoculum level 500 eggs/ plant and on the third assessment date (6 WAI) for 1000 eggs/ plant inoculum level. Harvest date affected galling in sugar beet roots (P ≤ 0.001); there was a significant interaction of harvest date × variety (P ≤ 0.001), the increase of gall index was greater for variety Elan than for the other tested verities. Based on galled area index, the resistant and susceptible varieties could be separated successfully as early as 2 WAI. At the highest tested inoculation level (12000 eggs/ plant), but it couldn't be separate between the moderately resistant and the susceptible or between resistant and moderately resistant ones at P ≤ 0.001. Based on eggs per gram root, the four sugar beet varieties with three levels of resistance to M. incognita were separated at the inoculation rate of 8000 eggs/ plant by 6 WAI and at 500, 1000, 2000, 4000 and 12000 eggs/plant by 8 WAI. In addition to gall number, gall index, galled area index, eggs per gram root, egg mass number and egg mass index were also used to assess the resistance levels in the sugar beet varieties. Gall index was found to be the most sensitive method of all measures used for assessing resistance. Inoculum type i.e. eight thousand eggs and 2000 J2 did not result in significant differences in galled area index at the two investigated harvest dates. Plant age at time of inoculation affected gall development on the tested sugar beet varieties however; the effects on Av poly, Lados and M 9680 were not as great as on Del 939 and Elan. But, the same tested verities could not be separated into their appropriate resistance categories with inoculation at 0 and 40 day after planting. The importance of such study is the identification of a rapid method for assessing resistance in sugar beet varieties to root-knot nematodes, takes less than 100 days.