Aphid honeydew, a potential stimulant of Cochliobolus sativus and Septoria nodorum and the competitive role of saprophytic mycoflora

Abstract

The effect of aphid honeydew on fungal leaf saprophytes and pathogens was studied on wheat under controlled environmental conditions. When honeydew was added to the inoculum, the colonization of flag leaves by a mixed population of Sporobolomyces roseus, Cryptococcus laurentii var. flavescens, Aureobasidium pullulons and Cladosporium cladosporioides was highly stimulated. After an exponential growth period of about 6 days, the population densities of all showed a tenfold increase in the presence of honeydew. Honeydew added to the inoculum of Septoria nodorum or Cochliobolus sativus enhanced the degree of infection of wheat leaves by these pathogens 2.5–4 times. During the prepenetration development of C. sativus, honeydew stimulated spore germination, the formation of two germ-tubes per conidium and germ-tube growth. Addition of saprophytes to the inoculum, enriched with honeydew, reduced the stimulation of infection in one experiment only where high population densities were reached within one day, but application of honeydew 4–5 days before inoculation, always resulted in an abundant population of naturally occurring saprophytes which completely neutralized the honeydew effect. Carbohydrate determinations of washings from leaves treated with artificial honeydew, revealed a decrease in washable carbohydrates from 450 to 40 μg ‘glucose’ cm−2 leaf within 7 days, with a simultaneous increase in the number of saprophytes from less than 104 to 4.106 C.F.U. cm−2. The implications of these results for the interpretation of secondary effects of aphids on the yield of wheat are discussed.