'BORDEAUX'/'KUMULUS' REGIMES AND 'SURROUND' IN ORGANIC APPLE PRODUCTION IN NEW BRUNSWICK: IMPACTS ON APPLE SCAB, FRUIT RUSSETING AND LEAF GAS EXCHANGE

A field trial was conducted over two growing seasons in a ‘Ginger Gold’ apple orchard in Bouctouche, New Brunswick, Canada, to assess the efficacy of different regimes of two organic fungicides, ‘Bordeaux’ (Copper 53W at 2 kg•ha^-1 + hydrated lime at 10 kg•ha^-1) and ‘Kumulus’ (80% sulfur at 22.5 kg•ha^-1), in controlling apple scab (Venturia inaequalis) and minimizing fruit russeting. ‘Surround’ (95% kaolin clay at 50 kg•ha^-1), an organic crop protectant, was used for insect control; its impact on leaf gas exchange [net photosynthesis (Pn), stomatal conductance (g_s), intercellular CO₂ (Ci) and transpiration (E)] was evaluated. A very minor incidence of fruit damage due to scab coupled with a high incidence of fruit russeting characterized the first year of the study; the converse was observed in the following year. The fungicide treatments exacerbated the russeting incidence from 8% in the untreated Controls to levels upwards of 59%, and severity from 1% in the Controls to a proportion of between 8 and 11% in the first growing season, and reduced the incidence of scab by 91 to 99% in the second season. Differences in the extent of apple scab and fruit russeting between years can be attributed largely to variance in the weather conditions characterizing the growing seasons (in the case of scab) and the days on which fungicide applications were made (in the case of russeting). At conditions of saturated light conditions, leaf temperatures of 27 to 36ºC and leaf residue rates of up to 3.7 g•m^-2, ‘Surround’ was observed to have a positive effect on both Pn and E; these were found to increase on average by 2.0 µmol CO₂•m^-2•s^-1 and 1.2 mol H2O•m^-2•s^-1, respectively, for each g•m^-2 increase of ‘Surround’ coverage. This can be explained by a concomitant increase in g_s at higher levels of ‘Surround’ leaf residues.