Directions in sustainable agriculture : the control of apple black spot

Abstract

Recurring problems within our agricultural environment suggest that the resource base is being degraded at a rate unacceptable to future generations. Current methods for controlling apple black spot are identified as an example of the problems we face and what is being done to rectify them. In particular, the use of fungicides is singled out, but it is acknowledged that entire management regimes are at fault. The move towards more sustainable production systems is seen as highly desirable for New Zealand from both environmental and financial perspectives. The marketing of organic apples to a world increasingly conscious of pesticide residues in food is seen as the obvious solution. The problem is a lack of suitable cultivars and techniques to control problems like Venturia inequalis, apple black spot fungus.

Research was undertaken to evaluate the prospective control techniques of urea and fish fertilizer applications to enhance the decomposition of the leaf litter and break the lifecycle of the fungus. This initially involved an evaluation of the sites of fungal overwintering within the leaf. Results show the lignified tissues of the petiole and midrib posses much smaller areas of disease lesion in comparison to the non-lignified lamina. The smaller area of lesion on the lignified tissue may assume greater epidemiological significance if it were to resist decay by urea or fish fertilizer. Subsequent experiments show that this tissue is capable of producing similar amounts of inoculum per area of lesion as that on the non-lignified tissues of the lamina. Autumn leaf dipping with fish fertilizer and urea was shown to reduce of the amount of spring primary inoculum by 12 and 80% respectively. The mode of action of urea was shown to be that of enhanced decomposition and an inhibition of fungal reproductive processes. The effect of urea over that of fish fertilizer precludes the latter as an effective option for disease control within a sustainable production system.