264 Percival and Haynes: The Influence of Calcium Sprays to Reduce Fungicide of apple scab on leaf and fruit tissue, and investigate the rela- tionship of foliar calcium content within resistant, intermediate, and sensitive apple species on resistance against apple scab to determine if analyzing foliar calcium concentrations provides a means of screening new cultivars for their scab sensitivity. MATERIALS AND METHODS Field Experiment The apple trial site consisted of a 1.5 ha block of apple (Malus cv. Crown Gold) interspersed with individual trees of Malus ‘Red De- licious’ and ‘Gala’ as pollinators. Planting distances were based on 3 m (9.9 ft) by 3 m (9.9 ft) spacing. The trees were planted in 1979 and trained as an open leader (i.e., bush shaped tree by periodically pruning the central leader). All experimental trees had an average height of 2±0.2 m (6.6±0.6 ft) with mean butt di- ameters of 33±5 cm (13.2±2 in) at 45 cm (17.7 in) above the soil level. The trial site was located at the University of Reading Shin- field Experimental Site, University of Reading, Berkshire (51°43 N, -1°08W). The soil was a sandy loam containing 4%–6% or- ganic matter, pH of 6.1, available P, K, Mg, Na, and Ca were 55.3, 702.4, 188.2, 52.9 and 1888 mg per liter (0.0001, 0.004, 0.002, 0.0001, 0.02 oz per gallon). Site management consisted of a 2 m (6.6 ft) wide, weed-free strip beneath the trees maintained with glyphosate (Roundup; Green-Tech, Sweethills Park, Nun Monk- ton, York, UK). Trees were irrigated with under-tree impact sprin- klers at 10-day intervals throughout the growing season. No sup- plementary fertilization was applied during the trials. A minimal insecticide (deltamethrin, product name Bandu, Headland Agro- chemicals Ltd, Saffron Walden, Essex, UK) program was applied every two months during the growing season commencing in May 2006 (Nicholas et al. 2003), a standard practice followed at the University of Reading experimental site for orchard pest control. All sprays were applied using a Tom Wanner Spray Rig sprayer at 40 ml (1.4 fl oz) deltamethrin per 100 L (26.4 gal) of water. Trees were sprayed until runoff, generally 3.5 L (0.9 gal) per tree. Calcium Treatments A randomized complete block was utilized in the experimental de- sign. Ten treatments included seven calcium products (calcium chlo- ride, calcium sulfate, calcium nitrate, calcium nitrate borate, calci- um hydroxide, calcium metalosate, calcium magnesium complex) (United Agri-Products, Alconbury Weston, Huntingdon, UK), one Silwett L77 (80% polyalkylene oxide, 20% allyloxypolyethylene glycol methyl, ether plant penetrant; De Sangosse, Swaffham Bul- beck, Cambridge, UK), one conventional fungicide (penconazole), and a water control. These 10 treatments were assigned to eight sin- gle-tree replications, totaling 80 observations per response variable. Calcium sprays were applied using a 5 L (1.32 gal) pump-ac- tion sprayer and trees were sprayed until runoff. Single-row bor- ders prevented overspray of calcium treatments and double-row borders prevented calcium drift. Calcium treatments were applied at bud break (March 11, 2006 and March 22, 2007), 90% petal fall (May 13, 2006 and May 27, 2007), early fruitlet (June 2, 2006 and June 7, 2007), and two weeks after early fruitlet (June 16, 2006 and June 21, 2007). Each calcium spray was calibrated so each tree received 2.5 g (0.09 oz) calcium per spray [10 g (0.75 oz) total]. Poor absorption and uptake of foliar applied calcium is a wide- ly recognized problem within the orchard industry. Consequently ©2009 International Society of Arboriculture calcium sprays are routinely applied in combination with a plant penetrant such a Silwett L77. Silwett L77 was applied singly and in combination with all calcium fertilizers at the manufacturers recommended rate, 5 ml (1 tsp) per ten liter (2.6 gal). A com- parative evaluation of the synthetic fungicide penconazole (10% emulsfiable concentrate), a protectant triazole fungicide with an- tisporulant activity commercially used for apple scab control, was conducted by spraying trees at the recommendation the manufac- turers (Syngenta Crop Protection UK Ltd, Whittlesford, Cam- bridge, UK), at the rate of 1.5 ml (0.045 fl oz) per liter of water. Leaf and Fruit Scab Severity Historically, the apples heavily suffered from apple scab infec- tion on an annual basis. As a result, before the trial commencing in 2006 and 2007, trees were inspected in September 2005 and 2006; and only those trees with 50%–80% of leaves affected, se- vere foliar discoloration, and subsequent scab infection were used. During the trials, assessment of scab severity of leaves and fruit commenced each September. Leaf scab severity of each tree was rated using a visual indexing technique and ratings on the scale: 0 = No scab observed; 1 = less than 5% of leaves affected and no aesthetic impact; 2 = 5%–20% of leaves affected with some yel- lowing but little or no defoliation; 3 = 21%–50% of leaves affect- ed, significant defoliation and/or leaf yellowing; 4 = 51%–80% of leaves affected, severe foliar discoloration; 5 = 81%–100% of leaves affected with 90%–100% defoliation. Scab severity on fruit was calculated on the scale: 0 = no visible lesions; 1 = <10% fruit surface infected; 2 = 10%–25% fruit surface infected; 3 = >25%–50% fruit surface infected; 4 = >50% fruit surface infected. The individual ratings for each tree in each treatment were used as a measure of scab severity for statistical analysis. Leaf scab severity ratings used in this study were based on UK and Ireland market standards for fungicide evaluation of scab control (Butt et al. 1990; Swait and Butt 1990). Fruit scab severity was based on a scale used by Ilhan et al. (2006). Scab severity ratings were under- taken by three independent BASIS (British Agrochemical Stan- dards Inspection Scheme) qualified crop protection specialists. Chlorophyll Fluorescence Prior to measurement, ten leaves per tree randomly selected throughout the crown were adapted to darkness for 30 min by attaching light exclusion clips to the leaf surface and chloro- phyll fluorescence was measured using a HandyPEA portable fluorescence spectrometer (Hansatech Instruments Ltd, King’s Lynn, UK). Measurements were recorded up to one second with a data acquisition rate of 10 µs for the first two ms, and of one ms thereafter. The fluorescence responses were induced by a red (peak at 650 nm) light of 1500 µmol m-2 s-1 photosyn- thetically active radiation (PAR) intensity provided by an array of six light-emitting diodes. The ratio of variable (Fv = Fm-Fo) to maximal (Fm) fluorescence (i.e., Fv/Fm where Fo = minimal fluorescence) of dark adapted leaves were used to quantify the detrimental effects of fungal infection on tree vitality. Fv/Fm is considered a quantitative measure of the maximal or potential photochemical efficiency or optimal quantum yield of photosys- tem II (Willits and Peet 2001) and consequently the most popular index used as a measure of plant vitality and early diagnostic of stress (Meinander et al. 1996). The ten leaf values per tree were pooled to provide a mean for each tree for statistical purposes.
September 2009
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