Development of a fungicide spray action threshold to help reduce Captan residue levels on fruits

Project code PRR06-910

Project lead Vincent Philion - Institute of Research and Development in Agroenvironment

Objective

To devise and implement a summer spray program that minimizes the use of the fungicide captan on apple trees

Summary of Results

Background

Captan is a fungicide commonly used by apple growers because it is affordable and is an option for control of both summer diseases and pin-point scab infections. Captan sprays directed against apple scab are typically applied in early spring to coincide with rainy periods during primary infection by fungal ascospores. Adequate suppression of ascosporic infections can result in a relaxed spray schedule up until harvest. Unfortunately, very low levels of primary infection on foliage can result in unacceptable scab levels on fruit when weather conditions in summer and early autumn are conducive to apple scab.

The purpose of this project was to help identify situations in which captan sprays can be safely omitted and to determine the optimal spray program needed to protect the fruit.

Approaches

Experiments were carried out between 2006 and 2009 in an experimental orchard in Quebec. In the apple scab spray threshold experiments, a 4 year-old McIntosh orchard (2200 trees/ha) was split into sectors of different scab levels and each sector was further split into plots treated with either: 1 - no summer fungicide application, 2 - current grower standard, or 3 - sprays based on a proposed threshold. Grower standard captan sprays were applied every 14 days or when 1.5 inches of rain had accumulated, regardless of scab levels. Scab levels were measured weekly by randomly sampling 10 shoots or clusters on 5 trees per plot in each sector.

In the captan residual efficacy experiments, a mature standard McIntosh orchard (120 trees/ha) was used. Normal fungicide sprays were initially applied to maintain apple scab at a low level, but no sprays were applied after June 1st. Twice weekly starting in late June, randomly selected trees were marked and sprayed once with captan and then inoculated with apple scab fungus in July. Uninoculated and unsprayed controls were maintained for each inoculation date. Apples were harvested in early September and stored at 4°C. Apple scab severity was evaluated after storage.

Results

All three summers during the course of the study sustained extended rainy periods, precluding any analysis of the spray threshold method under dry conditions. Overall, results were consistent across the three years of the project, demonstrating that in plots with scab levels below the proposed threshold in June, disease increase in August was not commercially acceptable. Although fruit scab at harvest remained low, scab observed after 12 weeks of storage was consistently above thresholds. Therefore, sprays applied once the threshold was reached were insufficient to maintain adequate control, indicating that the proposed threshold of 5 scabby leaves per 100 shoots in June is not conservative enough for a sensitive cultivar like MacIntosh during extended periods of rain. It was also noted that lowering the threshold to 1 scabby leaf per 100 shoots dramatically increases sampling time, and the cost of this added scouting may not warrant the potential benefits.

Despite these limitations, a new proposal for management of fruit scab was put forward, based on the concept of an extended pre harvest interval during which no fungicides would be needed. In this new approach, instead of stopping sprays when primary scab is over, growers would maintain minimal fungicidal coverage until a "summer spray stop date". The proposal received considerable support from risk-averse consultants. Three seasons of wet, scab-conducive weather illustrated that this approach could be used to maintain fruit quality, but did not reduce pesticide risks. Further work on this threshold in drier season would be needed to determine whether the spray action threshold would reduce pesticide risk while at the same time controlling apple scab to commercially acceptable levels.

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