Importance of Fungal Pathogens In Stunting of Lonren Cotton Lines Grown In Reniform Nematode-Infested Fields

The LONREN-1 and LONREN-2 Cotton Lines are virtually immune to reniform nematode (Rotylenchulus reniformis) in controlled inoculation studies.  Populations in soil are often reduced by 95 to 98 percent in two months.  Yet when planted in naturally nematode-infested fields these lines often show severe stunting, compared to their nematode-susceptible sibs, at two to three weeks after planting.  The affected plants show symptoms typical of fungal root rots.  A wide array of fungal pathogens was isolated from the roots.  We compared the ability of three fungicide drenches, Terrachlor (500ppm, a.i.) Terrazole (300ppm, a.i.) and Benlate (100ppm, a.i.), to prevent stunting in reniform-infested soils from Weslaco and Snook, Texas; Baton Rouge, Louisiana; and Arkansas.  Benlate almost completely prevented stunting in all soils.   Terrazole initially caused stunting in susceptible sib plants, but reduced it in Lonren plants in the Louisiana and Weslaco soils.  Terrachlor either increased stunting or had no effect.  None of the fungicides significantly reduced nematode populations.  In controlled inoculation studies Thielaviopsis basicola, Pythium ultimum, Pythium aphanodermatum and Rhizoctonia solani caused more severe root rot of Lonren lines in the presence of reniform nematode than in the absence of the nematode.  Alternaria, Diplodia, Fusarium, Macrophomina and Phoma species were not favored by the nematode.  These results indicate that the hypersensitive reaction to the nematode that results from the Ren^lon gene increases susceptibility to necrotrophic fungal pathogens that cause root rots.  Implications for use of the Ren^lon gene will be discussed.