Fungicide Application

Apple scab (Venturia inaequalis) is the principal disease affecting Bramley orchards in Northern Ireland. Up to 14 sprays of protectant fungicide can be applied throughout the season from mid-March until the end of August. Both the beneficial mite, Anystis baccarum and the pest mite species apple rust mite (Aculus schlechtendali) could be affected either directly via contact with the fungicide or, depending on the effectiveness of the fungicide in controlling scab, indirectly through quality of host leaves. Anystis baccarum is a known predator of apple rust mite capable of controlling its population numbers (Cuthbertson et al., 2003).

To detect the effect of the three most commonly used fungicides in the Bramley orchards – captan/penconazole, mancozeb and dithianon – on A. baccarum and apple rust mite populations, trees were randomly chosen within an orchard and sprayed via an electric knap-sac sprayer. The fungicides were applied every 10 days from 1 April until 26 July 1999. Numbers of A. baccarum were assessed by a three-minute visual count twice weekly, while apple rust mite numbers were assessed weekly throughout summer and early autumn. Scab control assessments were taken for each treatment along with total crop weight at harvesting time.

Highest rust mite numbers occurred in the mancozeb treatment (Figure 1), this same fungicide along with captan/penconazole had the most harmful effect on the beneficial mite A. baccarum (Figure 2). Dithianon was non-toxic to A. baccarum. Mancozeb gave the best scab control in this trial, and therefore the best leaf quality and consequently the highest yield at the end of the season (Table 1).

Figure 1. The effect of fungicides on apple rust mite (Aculus schlechtendali).

Figure 2 The effect of fungicides on the beneficial mite Anystis baccarum.

Table 1. Mean apple weight per six trees in each fungicide treatment.

This fungicide trial has shown that there would appear to be a link between leaf quality and numbers of apple rust mite. Within an Integrated Pest Management (IPM) programme what is required is a fungicide that gives adequate scab control yet is non-toxic to beneficial species (Cuthbertson and Murchie, 2003), which are therefore allowed to survive and control pest species.

Cuthbertson, A.G.S., Bell, A.C. and Murchie, A.K. (2003) The impact of the predatory mite Anystis baccarum on apple rust mite (Aculus schlechtendali) populations in Northern Ireland Bramley orchards. Annals of Applied Biology 142: 107-114.

Cuthbertson, A.G.S. and Murchie, A.K. (2003) The impact of fungicides to control apple scab (Venturia inaequalis) on the predatory mite Anystis baccarum and its prey Aculus schlechtendali (apple rust mite) in Northern Ireland Bramley orchards. Crop Protection 22: 1125-1130.