A Search for More Durable Resistance
In this second year our experiments to meet each of our objectives are well underway. The split-root experiments are set to determine if Meloidogyne arenaria pt. Harmony can physiologically alter Harmony rootstock thereby permitting normal root knot populations to enter Harmony roots. Our two-year evaluation of ectoparasitic nematodes feeding on 110R, 101-14, 5BB, Boerner, and Ramsey x Schwarzmann hybrids will complete this winter. We can already state that R x S-2 is too good a host for X. index but another R x S-9, which has low to medium vigor, continues to be a poor host for X index. We also continue to move these rootstocks and the USDA rootstocks into field trials. To determine resistance durability we have been observing and quantifying the resistance mechanisms of Freedom in the presence of different root knot nematode populations. A hypersensitive response (HR) is the browning of cells and tissues that often occur when a resistance mechanism is activated. Essentially the plant kills its own cells to defend itself from an invading pest. An HR can activate within 30 minutes of plant invasion. Infection by root knot nematodes does result in an HR in Freedom at site of penetration, during female development and during egg laying. It also occurs when nematodes attempt to enter some places other than just behind the root tip. The HR is also visible when the aggressive M. arenaria pt. Freedom is the infection nematode. The HR in Freedom involves only a few cells but can occur behind the root tip, along the differentiating vascular area and other locations along the root surface. The HR for 6-19B, 10-17A, R x S-2, and R x S-3 also occurs at all these locations but at each location involves 10 to 100 times the amount of plant tissue. We have not adequately studied 10-23B at this point in time and its mechanisms appear different. The new roots of the resistant stocks can actually become quite browned internally as compared to Freedom roots when under nematode attack. One of our concerns with HR has been that killing of plant cells could certainly result in slow plant growth until the pest population is depleted. However, the grape rootstock that promotes the greatest HR is 10-17A and to date we have never seen it with a lack of vigor. The 6-19B rootstock is slow to get started in sandy soils and it also exhibits extensive HR.