RELATIONSHIP BETWEEN SUNFLOWER DEVELOPMENT AND THE ONSET OF STEM ROT INDUCED BY SCLEROTINIA MINOR

Sclerotinia stem rot, caused by Sclerotinia minor, has not been report ed in sunflower (Helianthus annuus) during vegetative growth. During a n investigation of disease development in the highly susceptible inbre d line, PacF2582, the disease was first observed in the field at bud d evelopment. In glasshouse studies with inoculated soil, onset of disea se was delayed until late vegetative growth. Direct inoculation of roo ts with disks of mycelium or with pregerminated sclerotia confirmed th at sunflowers are susceptible to infection during the vegetative growt h stages. Root measurements found that the rate of increase in root dr y weight was maximum between the early and midbud stages in field-grow n plants (230%/week) and between the 6- and 10-leaf stages in glasshou se plants (240%/week). Synchronisation of disease onset with prolifera tion of lateral roots suggests that active root growth is required to initiate infection.