Reduction of primary inoculum of apple black spot by treatment of apple leaf litter

Abstract

Leaf treatments for control of apple black spot, caused by Venturia inaequalis, were investigated. These included methods for reducing development of the overwintering stage of the disease in apple leaf litter, thereby reducing primary inoculum in spring. Treatments to enhance leaf degradation and to reduce numbers of ascospores included application of a range of fungal isolates and leaf amendments, as well as use of different orchard ground management systems.

A laboratory screen of 59 fungal isolates from Alternaria, Fusarium, Aureobasidium, Phoma, Chaetomium, Trichoderma, Coniothyrium, Ulocladium, Epicoccum genera and white rot Basidiomycetes, by growth rate on sterilised disks of senesced apple leaves, identified the most effective 18 isolates. The ability of these isolates to degrade leaf disks, over a 9-week incubation period at 10 and 20ºC, was investigated with four leaf amendments. The four most effective of the isolates, Trametes versicolor (WR1), a Phoma sp. (237R), an Epicoccum sp. (E062) and a Chaetomium sp. (CG5) caused 53.6, 39.1,36.5, and 23.0%, respectively, reduction in leaf disc weight. The best leaf amendments, urea and Bio-Start™, contributed 29 and 20%, respectively, to leaf weight reduction. These treatments were selected and tested in a field trial.

The field trial was established in apple orchards at two sites, Hawkes Bay and Lincoln, to investigate the effects of leaf shredding, fungal isolates and leaf amendments on apple leaf litter which remained on the orchard floor from July to November 2001. At Lincoln, leaf weight losses for shredded and whole leaves were 10.6% and 7.8%, compared to Hawkes Bay where they were 10.3% and 6.3%, respectively. The effects of the fungal isolates on leaf weight was significant, with the most effective isolates, Epicoccum sp. (E062) at Lincoln and Chaetomium sp. (CG5) at Hawkes Bay, causing 2.7 and 4.5%, respectively, greater weight loss than the water control. Although the relative weight losses were small (but significant), visual observations also showed that leaf laminae were degraded, leaving the much heavier, lignified components. Weight measurements may also have been affected by the amount of soil and debris that could not be removed from the leaf bags.

The leaf amendments, urea and Bio-Start™, did not have significant effects on leaf weight loss but did have significant effects on numbers of ascospores released with urea reducing ascospore numbers from Hawkes Bay and Lincoln to 20.7% and 36.5% of the water controls, respectively. The effect of fungal isolates on ascospore numbers was not significant, although Epicoccum sp. (E062) and Chaetomium sp. (CG5) were associated with the lower numbers of ascospores in both sites. The most effective treatment overall was Chaetomium sp. (CG5) in combination with, urea, which reduced ascospore numbers to 17.7% and 14.7% of the nil fungus/water controls in Lincoln and Hawkes Bay, respectively.

An orchard understorey trial was carried out to study the effect of seven orchard ground management treatments, bare ground and polythene covered plots, as well as the vegetation covers, short grass, long grass, alyssum, perennial buckwheat and lupin. These created different canopies for obstructing ascospore flight, and different micro environments at soil level, where the apple leaves were sited. However, the results were inconclusive as the cover crops planted did not establish well and there were too few ascospores produced to determine their effects.

The effects of the treatments of leaf amendments and fungal isolates, especially urea in combination with Chaetomium sp. (CG5) and Epicoccum sp. (E062) for reduction of ascospore numbers appears promising and needs further study, which could be done with improvements in methodology.