Arboriculture & Urban Forestry 47(4): July 2021 a spectrophotometer (PU8800 Pye Unicam) after extraction with 80% v/v aqueous acetone (regression equation = 5.82 + 0.063x; r2 adjusted = 0.94, P < 0.001) (Lichtenthaler and Wellburn 1983). Ten leaves per tree randomly selected throughout the crown were used for measurements and the mean calculated per tree for statistical purposes. Fruit Yield and Crown Volume Yield per tree was determined by weighing all fruit on each tree at harvest and dividing by the number of trees per treatment. Crown volume (Cv ) was estimated from the crown width (D) and crown depth (L) using the paraboloid form of the crown (Kupka 2007): Data Analysis Data were analysed using Genstat 13. Levene’s (1960) test was used to determine the homogeneity of vari- ances, and data were transformed (log [y + 0.5]) when necessary. Visual severity index data were trans- formed using arcsine (√[y/100]). Treatment signifi- cance from controls was separated using Fisher’s least significant difference (LSD) at the P < 0.05 level. Limpel’s formula as described by Richer (1987) was used to describe synergistic interactions between the reduced dose of penconazole and both calcium and silicon fertiliser combinations. The presence of synergism between Ca and Si fertiliser and fungicide was determined by using Limpel’s formula: where Ee is the expected effect from additive responses of two inhibitory compounds, and X and Y are the per- centages of inhibition of each compound used alone. If the combination of the two agents produces any value of inhibition greater than Ee , then synergism exists (Lorito et al. 1993). RESULTS Container-Grown Stock: Guignardia Leaf Blotch Irrespective of Ca, Si, and penconazole combination, and whether applied at full or half strength, no leaf phytotoxic effects were recorded (data not shown). 173 Naturally occurring outbreaks of Guignardia leaf blotch were recorded on foliage of control trees as indicated by a leaf severity rating of 3.5 and 3.2 at the end of the 2016 and 2017 growing seasons, respectively (Table 2). The greatest reductions in Guignardia leaf blotch sever- ity on leaves was recorded following 6 repeat applica- tions of the synthetic fungicide penconazole at full strength, where no leaf blotch was recorded in the 2016 and 2017 trials (Table 2). Likewise, 6 repeat applications of the synthetic fungicide penconazole at full strength had the most positive effect on leaf chlo- rophyll content and crown volume compared to all other treatments (Tables 3 and 4). Limited efficacy against Guignardia leaf blotch was recorded when penconazole at half strength (0.5 mL L-1 ), Ca metaso- late, and potassium silicate were applied 3 times during a growing season. In these instances, leaf blotch severity symptoms (Table 2), leaf chlorophyll content (Table 3), and crown volume (Table 4) were statisti- cally comparable to water-treated controls. Combina- tions of penconazole at half strength (0.5 mL L-1 ) with either Ca metasolate or potassium silicate, however, when applied 3 times during a growing season, in most instances, significantly reduced leaf blotch severity symptoms and significantly increased leaf chloro- phyll content and crown volume compared to water- treated controls (Tables 2, 3, and 4). These effects were additive, not synergistic according to Limpel’s formula. Three spray applications of penconazole at full strength (1.0 mL L-1 ), tetra ethyl silicate, and Ca chloride, singly and in combination, significantly reduced leaf blotch severity symptoms and signifi- cantly increased leaf chlorophyll content and crown volume compared to water-treated controls (Tables 2, 3, and 4). Improved efficacy (reduced leaf blotch severity, increased leaf chlorophyll content and crown volume) was recorded using combinations of pen- conazole at full strength + tetra ethyl silicate or pen- conazole at full strength + Ca chloride compared to each product applied singly (Tables 2, 3, and 4). Sim- ilarly, 3 spray applications of penconazole at half strength (0.5 mL L-1 ) + tetra ethyl silicate or penconazole at half strength + Ca chloride had greater efficacy compared to spray applications of penconazole at half strength alone. These effects were primarily additive, with one exception: penconazole at half strength + Ca chloride on leaf chlorophyll content, where a synergistic effect based on Limpel’s formula was recorded (Table 3). ©2021 International Society of Arboriculture
July 2021
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