ASPECTS OF RESISTANCE IN DEEP-WATER RICE TO THE STEM NEMATODE DITYLENCHUS-ANGUSTUS

A seedling based technique for screening for resistance to D. angustus in deepwater rice was developed and used to examine components of var iability in host suitability. The techniques mimicked natural infectio n from water and achieved 100 % infection of susceptible check cv. NC4 92. Approximately 10% of inoculum invaded seedlings, the infective sta ges being predominantly J3, J4 and adult. Reproduction of D. angustus was rapid on susceptible plants (period of life cycle was 10-20 days a t 30-degrees-C). Symptom expression and nematode multiplication were s tudied on a range of resistant and susceptible deepwater rice cvs and lines. Resistance is in part conferred by a rapid necrotic response to feeding in the host. The response was exhibited by some plants of lin es, namely, CNL 319, Bazail 65, Rayada 16-02, Rayada 16-03 and Rayada 16-06 to Rayada 16-09, which, in the field, have been consistently res istant across locations and years. The response is qualitatively diffe rent from the well known susceptible response and provides a basis for genotypic selection. The relationship between water depth and seedlin g stature appears to be important in determining infection and early s ymptom development. D. angustus invades primarily at the water surface so submergence of the leaf sheath delays infection. Symptoms develop more quickly if the water level during infection is adjacent to, or ju st below, the collar at the top of the leaf sheath. The expression of symptoms and hence early damage is delayed in shallower water.