Control of Primary Infections of Subterranean Clover Red Leaf Virus, a Luteovirus, in a Broad Bean Crop With the Synthetic Pyrethroid Deltamethrin.

Abstract

Australasian Plant Pathology Vol. 13 No.4 December 1984 further pods after the symptoms of Infection become apparent (4). The greatest losses have occurred in spring-sown crops. These emerge either during, or Just prior to, the main peak of aphid flight activity and are therefore particularly susceptible to infestation by alate aphids. Alates of the vector of SCRLV (Aulacorthum solani) mainly originate from the most important source of infection, white clover plants (Trifolium repens) in perennial pastures. Primary infections resulting from the activity of vector alatae have not been amenable to control with conventional aphtcldss such as the organophosphates in the past (5). The approach to control has therefore been to advance the sowing date from the spring to late autumn or early winter (4). Although few aphids are flying during the early growth and development of these crops, a few foci of infestation and infection are generally established within them soon after they emerge. Considerable spread from these foci may occur due to movement of apterous aphids but these secondary infections can be controlled with aphicides such as demeton-S-methyl (5). The necessity to sow in the late autumn/early winter period can cause problems in Tasmania. Low temperatures and wet conditions during this period sometimes cause long delays in land preparation and sowing. These conditions also favour the development of chocolate spot, caused by Botrytis fabae, which is another important disease of broad bean in Tasmania (7). A recent report Indicated that the synthetic pyrethrord deltamethrin had a particularly rapid knockdown action or repellant effect against aphids; it substantially reduced infection of plants with a persistent, a sermpersistent and a non-persistent virus transmitted by Myzus persicae (1). An experiment was therefore designed to test whether deltamethrin would control primary infections of SCRLV in spring-sown broad beans in Tasmania. Broad beans (cv. Cotes Dwarf Prolific) were sown at Forth in August 1983 in rows set 500 mm apart to establish 30 plants/m'. Plots measuring 2 m (4 rows) x 8 m were sprayed fortnightly from emergence until harvest with either deltamethrin (25% e.c.; 50 mi per 100 L) or with demeton-S-methyl (25% e.c.; 100ml/100 L); unsprayed plots were also included. The plots were randomized in blocks and replicated four times. Immediately prior to each spray application, counts were made of the number of plants with SCRLV symptoms in a buffered zone (6 m long) in the central two rows of each plot. Randomly selected plants were also sampled from each plot to assess aphid mtestation