Resistant Germplasm in Gossypium Species and Related Plants to the Reniform Nematode.

All commercial cotton varieties are susceptible to the reniform nematode (RN), Rotylenchulus reniformis Linford & Oliveira 1940. The parasites cause 30-60% crop loss in infested fields. Resistant varieties, if found, offer the best control for this nematode. The objective of this research was to identify RN resistant germplasm in cotton species and related plants, and to investigate host parasite relationships. The 200 plants tested for RN resistance were 111 entries of G. hirsutum, seven entries of G. herbaceum, 14 entries of G. arboreum, six entries of G. barbadense, 33 entries of wild Gossypium spp., 22 entries of Hibiscus spp., and seven other genera in the Malvaceae. Each plant was inoculated with 2000 RN. Host resistance was based on RN egg production/gram of root after 35 days. Gossypium hirsutum cv. Deltapine 16 was used as the susceptible check. Resistance was found in 18.5% of the entries. Gossypium longicalyx was immune, and Sida rhombifolia was a non-host. High resistance was found in G. stocksii, G. somalense. and G. barbadense Texas 110. Other cotton lines with potential value for breeding RN resistance were G. herbaceum P.I. 408775; G. arboreum P.I. 417895, P.I. 417891, CB 3839; G. hirsutum Texas 893 and La RB 15702. Seventy-four percent of the Hibiscus spp. tested were resistant. These and related plants with RN resistance may be useful in crop rotation with cotton and in inter-generic gene transfers to cottons to reduce RN populations. Female development and egg production on the plants reflected host resistance. Healthy females and large egg masses were observed on susceptible plants and degenerated females and small egg masses were observed on resistant plants. Females penetrating immune G. longicalyx never matured to kidney shape. Histopathological root sections of RN infected susceptible plants showed the endodermis and pericyclic cells as feeding sites. The pericycle hypertrophied into syncytia. Syncytial cytoplasm was dense and granular. Nuclei and nucleoli were enlarged. Cell wall dissolutions may occur between hypertrophied cells. Resistant plants may exhibit hypersensitive reactions. The endodermal and pericyclic cells at feeding sites were collapsed or killed. Heavily stained deposits accumulated in and around these cells. Hibiscus lasiocarpos and Cienfuegosia drummondii were extremely RN susceptible. Their roots were injured by the parasites and therefore supported low RN egg production. Immune and resistant germplasm to RN were identified in Gossypium spp. for the first time. This knowledge may be used by plant breeders to develop RN resistant cotton varieties and to aid further genetic and inheritance studies in cotton