USE OF SHADING AND THE PSII INHIBITOR METAMITRON TO INVESTIGATE THE RELATIONSHIP BETWEEN CARBOHYDRATE BALANCE AND CHEMICAL THINNER ACTIVITY IN APPLES

The effect of shade and foliar sprays of the PSII inhibitor metamitron on activity of chemical thinning agents applied to ‘Morgan Spur Delicious’/MM.111 apple trees (Malus ×domestica) was investigated in replicated field experiments over three consecutive years. Shade and metamitron treatments were used to create a transient carbohydrate stress beginning on the day of thinner application at petal fall or when the mean fruit diameter was 10 mm. Daily maximum/minimum temperatures (°C) and total solar radiation (MJ/day) were used to estimate daily carbon balance using the Malusim model. Spraying trees with 350 mg/L metamitron at petal fall had no effect on fruit set, whereas application at the 10 mm average fruit diameter stage resulted in heavy fruit drop in 2011 and 2012, and complete elimination of fruit in 2010. Application of 600 mg/L carbaryl at petal fall reduced fruit set in 2010 and 2012, but had no thinning activity in 2011. Covering trees with a neutral density 80 percent shade cloth for five days did not increase the thinning activity of carbaryl at petal fall, but significantly increased the activity of 6-benzyladenine plus carbaryl applied at the 10 mm fruit diam. stage. These data suggest that apple fruit are insensitive to a carbo¬hydrate stress at petal fall, whereas a carbohydrate stress at the 10 mm fruit diameter stage can greatly increase activity of thinning chemicals. A thinning index based on average daily carbon balance for four consecutive days beginning on the day of thinner application did not explain yearly variation in thinning response.