"RESISTANT" VARIETIES:

DEFINITIONS

• Immune - plants do not allow nematode attack including initial root invasion.
• Resistant or nonhost - plants may be invaded by nematodes and may show damage but chemical or physical unsuitability of the plant will prevent population increases.
• Susceptible - plants allow normal nematode reproduction and may or may not tolerate nematode attack.
  • Tolerant host - plants are able to withstand nematode attack.
  • Intolerant host - plants are more likely damaged by nematode attack.

EXAMPLES

• Immune - root knot nematodes completely avoid roots of Blenheim apricots.
• Resistant or nonhost - Citrus is a nonhost for root knot nematodes: the nematode penetrates but fails to further develop in its life cycle. Nemaguard rootstock is resistant to all root knot nematode species currently present in California. The reistance mechanism of Nemaguard and other resistant cultivars does not restrict attack and penetration by root knot. However, the resistance mechanism is effective in halting nematode reproduction. Massive attack of young Nemaguard roots can cause visible growth reduction in the first year's growth, but if nematode reproduction is limited by controlling weed hosts, the resistance mechanism can be highly effective in the field. Some cultivars resistant to root knot nematode exist for some field and vegetable crops: for example, a number of tomato cultivars are resistant to Meloidodyne incognita, M. javanica, and M. arenaria; the cowpea cultivar Blackeye 5 and the lima bean cultivar White Ventura N are resistant to M. incognita and M. javanica, although they can be damaged by heavy infestation.
• Susceptible -
  • Tolerant host - Thompsom Seedless grape sometimes can be heavily galled by root knot nematodes, but once established it ususaly suffers light-to-moderate galling only. Apparently root knot nematodes have difficulty penetrating roots of the grapevine. Once inside, development is slowed, but relatively high egg production occurs. Field corn and wheat are also hosts for root knot nematodes, but are tolerant to damage and can yield well under moderate-to-heavy infection.
  • Intolerant host - Kiwis are highly susceptible to root knot nematode. Nematode penetration, development, and reproduction appear uninhibited and uninterrupted. Plant damage can be substantial unless procedures for avoidance of plant stresses are adhered to. Cururbit crops (squash, cucumbers, melons) are also highly susceptible to root knot nematodes.

Within nematology, the word resistance is used differently in different situations. The definitions provided here are from the University of California Phytonematology Study Guide. They are not agreed upon by all nematologists but are those that will be used in our discussion.

For example, root knot nematodes are not able to reproduce on nematode resistant tomatoes. However, alfalfa which is sold as nematode resistant may support substantial populations of nematodes and should not be used as a means to try to reduce nematode numbers in a crop rotation program. By these definitions, nematode resistant alfalfa would be a susceptible but tolerant host. When dealing with resistant varieties, one should always ask three questions: (1) which nematodes is the variety resistant to, (2) will nematodes damage the variety, and (3) will nematodes reproduce on the variety.

Although resistant varieties provide excellent protection, do not plant them year after year in the same field. Root knot nematode populations have the potential to overcome host plant resistance when exposed repeatedly to varieties with the same genetic background.

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