The effect of runoff on spray deposition and control of Alternaria brown spot of mandarins

Alternaria alternata pv. citri is the causal agent of Alternaria brown spot on tangerines and their hybrids and infects young leaves and fruit of all ages. In South Africa, repeated high volume (~9000 L/ha) fungicide sprays on susceptible cultivars are the only effective control measure of this disease. The effect of runoff on spray deposition and biological efficacy was largely unknown. The aims of this study were first to characterise spray deposition and runoff on fruit and leaves, and second to determine the effect of runoff on biological efficacy. Mature Nova mandarin leaves (upper and lower leaf surfaces), Valencia Late oranges and Eureka lemons were sprayed with different volumes of, or dipped in, a mixture of water and a yellow fluorescent pigment. Sprayed parts were illuminated under black light, visualised under a stereomicroscope and digitally photographed at 10x magnification. Quantitative and qualitative deposition assessment of the spray deposition was performed by means of digital image analyses. Hoerl regression curves were fitted to quantitative and qualitative deposition values on upper and lower leaf surfaces over spray volume (R²-values >0.95) and trends clearly indicated that deposition on young or mature leaves and fruit improved as spray volume increased, but only until the point of runoff was reached, thereafter deposition quantity and quality decreased. Deposition values following dip treatments were in all cases significantly subordinate to those of the best spray volumes. Mature upper leaf surfaces and Eureka lemon fruit generally retained less spray deposits than lower leaf surfaces and Valencia Late orange fruit, respectively. In order to determine the effect of runoff on biological efficacy of copper hydroxide against Alternaria brown spot, young Nova leaves were treated in a similar fashion and subsequently drop-inoculated with a virulent strain of A. alternata pv. citri and incubated for 3.5 days in moist chambers at 25°C. Biological efficacy of sprays followed a quadratic trend over spray volume and clearly demonstrated the detrimental effect of runoff on biological efficacy of fungicide sprays. Sigmoidal regression analyses of mean infection percentages against quantitative and qualitative deposition on upper and lower surfaces of young Nova leaves yielded very good fits indicating the correlation between biological and deposition data.