Fusarium yellows of sugar beet is caused by fungi in the F. oxysporum species complex (FOSC) and impacts the sugar beet industry from the field to post-harvest storage. The objectives of this work were to 1) identify pathogenic FOSC isolates collected in Michigan on sugar beet and assess their virulence levels and 2) to examine the infection process and to lay the groundwork for identification of potential histopathological differences between F. commune, a member of the FOSC and other FOSC members. The first set of objectives were addressed by isolating, identifying, and assessing virulence of Michigan FOSC isolates in a greenhouse assay using foliar and root severity ratings for Fusarium yellows. In comparison to the controls, of the 35 isolates screened in the greenhouse, 5.7% were classified as moderate virulence, and 60% were classified as low virulence, and 34.3% were non-pathogenic. The moderately virulent isolates will be of benefit to subsequent experiments and resistance screening trials targeted to manage Fusarium yellows in Michigan. For the second set of objectives, mature sugar beets were inoculated with an F. commune isolate and plants were collected every three days post inoculation through 18 days. The bottom half of the roots were fixed and stained for conventional and confocal microscopy. It was observed that F. commune initially colonized the root surface of the tap root and feeder roots. While penetration did occur in the feeder roots, the colonization of the vasculature of the feeder roots did not progress to the main tap root. On the tap root, penetration and colonization of the root interior occurred around the root tip. Hyphae subsequently grew into and traveled via the xylem in cambial rings and the stele from the root tip up to at least as far as the root groove. The knowledge acquired over the course of these experiments will help sugar beet breeding programs and growers make informed decisions on managing Fusarium yellows of sugar beet.
Read
