Characterization of Fusarium Root Rot Isolates from Sugar Beet by Growth and Virulence at Different Temperatures and Irrigation Regimes.

Fusarium root rot, caused by Fusarium oxysporum f. sp. betae is one of several root diseases damaging to sugar beet production in Texas. As a primary symptom, most isolates produce a severe tip rot on the distal end of the taproot, in addition to discoloration of vascular elements and wilting. Tip rot distinguishes Fusarium root rot from another F. oxysporum f. sp. betae-incited disease, Fusarium yellows, which induces vascular discoloration and wilting but no root rot. This study was conducted to further characterize a selection of five Texas F. oxysporum f. sp. betae isolates representing different vegetative compatibility groupings, symptom types (tip rot, and non-tip rot), and hosts. Radial growth at six temperatures was measured for each isolate in culture on half-strength potato dextrose agar. Significant growth differences were detected, indicating a substantial amount of variation among the isolates. Virulence of isolates was evaluated by inoculating 6-week-old sugar beet plants with a microspore suspension and transplanting them into infested field soil. The plants were incubated at 20 and 30°C in controlled temperature boxes within the greenhouse and grown under two different irrigation schedules. After 6 weeks, plants were harvested and assigned a root disease rating, and root and foliar dry weights were determined. Disease ratings among isolates at 30°C resulted in three isolates (all tip rot isolates) being severe, one mild, and one intermediate. At 20°C, only one isolate caused appreciable root damage. Irrigation treatments had no effect on disease incidence or severity. Significant differences in colony diameter growth and virulence among isolates at the two temperatures provide further evidence of variation among Texas F. oxysporum f. sp. betae populations. Results also suggest that the tip rot phenotype may be induced by some genetic factor unique to tip rot isolates. Therefore, the form name F. oxysporum f. sp. radicis-betae is proposed for those isolates from Texas causing a tip rot.