Characterization of Rhizoctonia solani AG 2 isolates causing bare patch in field grown tulips in the Netherlands

Abstract

During a spring survey in 1991, 130 isolates of R. solani were collected in 25 commercial flower bulb fields from diseased plants occurring in bare patches. On the basis of hyphal fusion frequency and pathogenicity to flower bulbs, tulip isolates were provisionally assigned to AG 2-t to distinguish these isolates from AG 2-1 isolates which were non-pathogenic to bulbs. Hyphal fusion frequency of a subgroup of 7 AG 2-t isolates was highly variable when paired with 7 AG 2-1 isolates (2-75%), thus making assignment of AG 2-t isolates to AG 2-1 inconclusive. The mean hyphal fusion frequency among AG 2-t isolates was 65% (±6%) indicating AG 2-t to be a relatively homogeneous group. Hyphal fusion frequency among AG 2-1 isolates was highly variable with a mean 51% (±25%) indicating AG 2-1 to be a heterogeneous group. The optimum growth temperature for AG 2-t and AG 2-1 isolates on malt peptone agar was 20-25 °C. The host range of AG 2-t and two AG 2-1 isolates comprised tulip, iris, hyacinth and lily at both 9 and 18 °C, and cruciferous, sugarbeet and lettuce seedlings at 18 °C. Six other AG 2-1 isolates were pathogenic to cruciferous seedlings, but not to any of the bulbous crops. The tested narcissus, Tagetes patula, tomato, potato, wheat, leek and maize cultivars were not susceptible to AG 2-t and AG 2-1 isolates. Statistical analysis using a proportional-odds model revealed significant differences in aggressiveness between R. solani AG 2-t isolates and differences in susceptibility between tulip and iris cultivars. At 18 °C, but not at 9 °C, isolates representing AG 2-2, AG 4, AG 5 and AG BI were pathogenic to bulbous crops. In addition to bare patch causing AG 2-t isolates, other anastomosis groups may cause disease in field grown tulips. For the development of optimal crop rotation schedules, the impact of bulb rot causing isolates under field conditions needs further study.