Interactions between Fusarium avenaceum and Hylastinus obscurus (Coleoptera: Scolytidae) and Their Influence on Root Decline in Red Clover

Abstract

Fusarium avenaceum [(Corda ex Fries) Saccardo] and clover root borer, Hylastinus obscurus (Marsham), have been implicated as causal agents in red clover ( Trifolium pratense L.) root decline which may lead to stand deterioration within 2 yr. Fumigated and nonfumigated plots of red clover were established in a field pasture with or without added phosphorus (P) and with or without added inoculum of F. avenaceum . Clover root borer infestations occurred naturally in this field setting. F. avenaceum infection did not exceed 23% of feeder root segments sampled during the 1985 growing season, and about 90% of the root sites were asymptomatic. No fungal infection or root borer infestations were detected in tap roots. Fusarium infection remained unchanged in feeder roots during 1986. Root injury by H. obscurus was positively correlated (r2 = 0.978) with enlargement of tap roots. Larger tap roots, which clover root borer adults preferred for oviposition, were the direct results of improved soil P levels. The fact that tap root infection by F. avenaceum was restricted mostly to sites of root damage by H. obscuros indicated that clover root borers could be the vector of F. avenaceum inocula and predispose tap roots to Fusarium infection during ingress. Therefore, red clover root decline is caused primarily by root borer injury which occurs severely in fields where soil P levels are high, and red clover growth is accelerated. F. avenaceum is not highly virulent in natural field settings and is of secondary importance in red clover decline.