Infection of cotton byfusarium oxysporum f.sp.vasinfectum as affected by water stress

Abstract

Since virulence ofFusarium oxysporum f.sp.vasinfectum (FOV) on cotton (Gossypium hirsutum) is enhanced when the fungus is cultivated in a saline environment, excessively saline water must not be used for the irrigation of cotton. However, the limitations thus placed on the available water resources may lead to conditions of enforced water stress for the plant. The present study investigated whether water stress affects the susceptibility of cotton to FOV. Groups of 2-month-old cotton plants of theFusarium-susceptible Coker 304 and the moderately resistant GSC 20 varieties were maintained without watering for varying periods immediately before or after being inoculated with FOV (15 plants per group, two replications). Watering was suspended for 3, 6, 12 or 24 days before inoculation, and for 3, 6, 12 or 15 days after inoculation. After inoculation the plants were maintained in a controlled environment with a 15,000 lux, 12-h photoperiod, at 28°/24°C D/N, 20% r.h. Xylem water potential was determined in a pressure chamber. Percent infected leaf area and date of onset of wilt were the parameters used to define severity of FOV infection. There was a consistent relation between low water potential in the xylem (-7 and -20 MPa) and severity of infection, particularly when the dry period occurred after inoculation. After exposure to the lowest post-inoculation water potentials, even variety GSC 20, which is normally moderately resistant, exhibited a fairly high percent infected leaf area. This should be taken into account when the cotton grower is faced with water shortages, especially during the period from branching to flower bud break.