Surveys of spraying practices for banana leaf disease control in New South Wales, and an assessment of disease control based on leaf retention at harvest in July

Abstract

Commercial 'Williams' banana plantations were surveyed at random on the North Coast of New South Wales during July 1990 and 1991, to determine spraying practices for leaf disease control and their efficacy in relation to leaf retention at harvest. A climatic index (CIP), based on temperature and wetness, indicated environmental conditions were conducive to leaf diseases caused by Mycosphaerella musicola and M. musae over summer and autumn 1990 but were less conducive in 1991, especially in autumn. In 1990, 5 of 44 plantations surveyed on the Mid North Coast (MNC) received no fungicide sprays; the remainder received 1-5 sprays of petroleum oil or oil mixed with propiconazole (mean 3.0 ¦ 1.3) in December-May. Two plantations were sprayed using misting machines, the other 37 were aerially sprayed. The average numbers of leaves remaining on plants with mature bunches ranged from 0.3 to 8.3 (mean 4.7 ¦ 2.2). A spray frequency model indicated that leaves per plant at harvest in July increased with numbers of sprays applied and decreased with increasing damage from wind and frost (r2 = 0.49; d.f. = 40; P<0.01). Another model based on the accumulated CIP from October to July (+CIP), adjusted for the timing and fungicidal effects of each spray, indicated that leaves per plant at harvest decreased with increasing +CIP, and wind and frost damage (r2 = 0.53; d.f. = 40; P<0.01). In 1991,4 of 38 plantations on the MNC and 3 of 32 on the Far North Coast (FNC) received no sprays; the remainder received 1-9 sprays of oil, oil + propiconazole, or oil + protectant fungicide mixtures. Fewer sprays were applied on the MNC in 1991 than 1990, but the timing of sprays remained unchanged. More sprays were applied on the FNC than MNC in 1991, and sprays were applied from December to July. Leaves per plant varied from 4.2 to 12.1 (mean 8.3 ¦ 2.0). The spray frequency model from 1990 predicted leaves per plant in 1991 poorly (r2 = 0.17; d.f. = 68; P<0.01) and with substantial bias, whereas, the CIP model predicted leaves per plant for the MNC and FNC well (r2 = 0.41; d.f. = 68; P<0.01) and without bias. Observations in 1991 on the 70 surveyed plantations indicated that leaf retention also decreased with increasing damage by burrowing nematode (Radopholus similis) and banana weevil borer (Cosmopolites sordidus), increasing latitude of the plantation, and decreasing electrical conductivity of the soil solution. These variables accounted for a further 21% of the variation in leaves per plant observed in 1991. Leaf retention was not significantly (P>0.05) related to soil pH or to spraying method (aerial v. ground).