Infection of sweetpotato fibrous roots by Streptomyces ipomoeae: Influence of soil water potential

Abstract

The effect of the matric component of soil water potential (Ψ m) on infection of fibrous roots of sweetpotato ( Ipomoeae batatas) cv. Jewel by Streptomyces ipomoeae and the influence of infection on water extraction by fibrous roots were examined. The severity of disease on fibrous roots was low in plants grown at constant Ψ m of 0, −1.0 or −2.5J kg −1 in non-fumigated or fumigated soils infested with S. ipomoeae. Disease severity increased with decreasing Ψ m and was greatest at Ψ m of −7.5 to −20J kg −1. Growth of S. ipomoeae in water-filled pores and subsequent infection may have been limited at Ψ m of 0, −1.0 and −2.5J kg −1. Root and shoot dry weights of sweetpotato were significantly lower in plants grown in infested soil than in non-infested soils at Ψ m between −5 and −20J kg −1, but were not affected by disease at Ψ m of 0, −1.0 or −2.5 J kg −1. The severity of disease on fibrous roots was low in plants drip-irrigated on a daily schedule, whereas the severity of disease on fibrous roots was significantly greater in plants irrigated on either a 4- or 6-day schedule. Total dry weights of roots were lower in plants grown for 4 weeks in infested than non-infested soil. However, total dry weights of roots were not affected by disease as compared to non-inoculated controls in plants grown for 8 weeks, thus suggesting that roots of cv. Jewel may be able to compensate for disease by production of additional root biomass in soil. Although root dry weight was not affected by disease in plants grown for 8 weeks, diseased plants extracted significantly less water from soil than healthy plants. Therefore, the effect of disease on water extraction from soil was not due solely to a reduction in root biomass. Limited growth of roots to inoculum in saturated soil, limited growth of the pathogen in saturated soil, or altered susceptibility of the host may explain the reduction of disease at high Ψ m.