Heat stability of resistance in selected tomato breeding lines against Meloidogyne incognita and M. javanica populations under elevated soil temperatures

In tomato, the only commercially available source of resistance to root-knot nematodes (RKN) is the Mi-1 gene that confers resistance to Meloidogyne incognita, M javanica and M arenaria. However, its effectiveness was limited at higher soil temperatures. A study was initiated with the objective to check the durability of the potential resistance genes found in some tomato breeding lines after screening in controlled glasshouse conditions <= 27 degrees C by exposing them to higher soil temperatures at 28, 32 and 36 degrees C for 24 and 48 h periods. The aggressive Jittu and Babile M incognita and Jittu and Koka M javanica populations originally collected from Ethiopia were used. When seedlings reached the four-leaf stage, each tube was inoculated with 50 freshly (< 24 h) hatched infective second-stage juveniles (J2). Immediately after inoculation, the seedlings were exposed continuously for 24 and 48 h in a warm water bath at 28, 32 and 36 degrees C, respectively. A control was kept separately in ambient temperature (24 degrees C 2 degrees C). The external ambient temperature and the soil temperature inside the tube while in the water bath were simultaneously recorded using a TESTO data logger. Temperature, tomato breeding lines and time had a significant effect on the number of J2 of Jittu and Babile M incognita and Jittu and Koka M javanica populations that penetrated the roots. The utility of the potential resistance found in the breeding lines during the controlled growth chamber resistance screening experiment was limited at higher soil temperatures, especially at 32 and 36 degrees C. At 36 degrees C there was no significant difference found on the mean number of penetrated J2 of Jittu and Babile M incognita and Jittu and Koka M javanica populations inside the roots of all the tested breeding lines compared to 'Marmande' (a susceptible control) after 48 h of heat exposure after inoculation. More J2 were found in the roots of the tested breeding lines after 48 h compared to 24 h heat exposure after inoculation for each soil temperature level tested and for both populations of M incognita. It is clear from our observations that local tomato breeding lines with resistance potential can be used when soil temperatures remain below 32 degrees C. Differences were observed between breeding lines depending on the RKN population used at higher temperatures and this knowledge can help in further optimising the development of sustainable resistance under local Ethiopian circumstances.