Ca. Engelkes et Ce. Windels, RELATIONSHIP OF PLANT-AGE, CULTIVAR, AND ISOLATE OF RHIZOCTONIA-SOLANI AG-2-2 TO SUGAR-BEET ROOT AND CROWN ROT, Plant disease, 78(7), 1994, pp. 685-689
Roots of cultivars Maribo Ultramono (MU, susceptible) and ACH 184 (mod
erately resistant) of sugar beet (Beta vulgaris) were inoculated with
two AG-2-2 isolates of Rhizoctonia solani (from sugar beet and pinto b
ean [Phaseolus vulgaris]) at 6, 8, 10, and 12 wk after planting. Two p
rocedures were followed to inoculate roots at four ages. Seeds were pl
anted on the same day and roots were inoculated at 2-wk intervals (con
secutive inoculations); and seeds were planted at 2-wk intervals and r
oots were inoculated on the same day (simultaneous inoculations) in 19
90-1991 field trials. From 2 to 8 wk after inoculation, roots were rat
ed for disease (0-7 scale) at 2-wk intervals. Root rot severity was ab
out two disease ratings higher at each evaluation in 1991 than in 1990
. In three of four trials, disease decreased as root age at time of in
oculation increased. In four trials, MU had a higher root rot rating (
4.5) than ACH 184 (3.6); and in three trials, the R. solani isolate fr
om pinto bean gave a higher root rot rating (4.9) than the isolate fro
m sugar beet (3.8). In culture, the former isolate grew faster by 0.01
-8.0 mm/24 hr at 25-35 C than the latter isolate. Average weekly air t
emperatures were between 25 and 35 C for at least 4 of 8 wk following
inoculations in the field. Overall, the least amount of Rhizoctonia ro
ot and crown rot occurred on the moderately resistant sugar beet culti
var as plant age increased.