A Search for More Durable Resistance
This report concludes a three-year study. The location and quantity of resistance mechanisms within Harmony, Freedom, Teleki 5C, 10-17A, 10-23B, 6-19B, RS-3 and RS-9 have now been microscopically observed as various root knot nematodes infected them. A resistance mechanism common to many plants is referred to as a host hypersensitive response (HHR). This is observable as the plant kills its own cells just ahead of an invading pest. The new rootstocks listed above exhibit five- to ten-times more HHR than Harmony or Freedom rootstock. All the new rootstocks listed above are also able to exclude our aggressive pathotypes of root knot nematodes which can devastate roots of Harmony and Freedom. The biochemical method each rootstock utilizes to kill these cells has not been studied and should be considered to be dissimilar among the rootstocks. All the new rootstocks listed above are known to possess broader nematode resistance than Harmony or Freedom. This study indicates that HHR in the new rootstocks can occur in abundance all along the surface, cortex and vascular bundles of the extending root system. We believe HHR to be the main source of “Broad Nematode Resistance.” It is also clear that HHR only occurs as a response to infective J2 of root knot nematode. Once the J2 have developed a feeding site and developed to J3, the HHR was never observed. Freedom and Harmony possess HHR but in limited amounts and locations. Invasion by seven or eight J2 at one time is adequate to overwhelm the mechanism, the juvenile feeds and a new and distinct pathotype eventually results for that rootstock. Teleki 5C exhibits minimal HHR but is able to starve some of the J4 and adult stages by limiting feeding site development (also results in none or smaller galls). This mechanism may not be too effective on the younger roots but can have value on older roots where offspring J2 attempt to take over feeding sites from their mother. 10-23B exhibits HHR as well as a mechanism that limits feeding site development. Of all the rootstocks we have studied we believe 10-23B to have the broadest and potentially most durable mechanisms for nematode resistance. 10-17A and RS-9 exhibit such an abundance of HHR that if other mechanisms also occur we were unable to detect them. Our two-year nematode rootstock profiles revealed that Boerner, 110R, 101-14, and 5BB do have resistance to specific nematode pests but do not possess the breadth of resistance like the new rootstocks listed above. An improved nematode susceptibility chart has been developed. Additionally, five and 10 year old rootstock trials that we have monitored have indicated weaknesses and strengths of the original two-year studies.