Abstract
A digital camera fitted with a hemispherical lens was used to generate canopy leaf area index (LAI) values for a banana (Musa spp.) field trial with the aim of establishing a method for monitoring stresses on tall crop plants. The trial in Uganda consisted of two cultivars susceptible to nematodes, a plantain, Gonja manjaya and an East African Highland banana, Mbwazirume, plus a nematode resistant dessert banana, Yangambi km5. A comparative approach included adding a mixed population of Radopholus similis, Helicotylenchus multicinctus and Meloidogyne spp. to the soil around half the plants of each cultivar prior to field planting. Measurements of LAI were made fortnightly from 106 days post-planting over two successive cropping cycles. The highest mean LAI during the first cycle for Gonja manjaya was suppressed to 74.8±3.5% by the addition of nematodes, while for Mbwazirume the values were reduced to 71.1±1.9%. During the second cycle these values were 69.2±2.2% and 72.2±2.7%, respectively. Reductions in LAI values were validated as due to the biotic stress by assessing nematode numbers in roots and the necrosis they caused at each of two harvests and the relationship is described. Yield losses, including a component due to toppled plants, were 35.3% and 55.3% for Gonja manjaya and 31.4% and 55.8% for Mbwazirume, at first and second harvests respectively. Yangambi km5 showed no decrease in LAI and yield in the presence of nematodes at both harvests. LAI estimated by hemispherical photography provided a rapid basis for detecting biotic growth checks by nematodes on bananas, and demonstrated the potential of the approach for studies of growth checks to other tall crop plants caused by biotic or abiotic stresses.
Highlights
Establishing damage levels from biotic stresses and the resultant economic loss to crops requires optimisation between assuring the reliability of the information gained and the effort and resources invested in its acquisition
Damage and economic thresholds based on nematode densities recovered from roots have been determined for Radopholus similis, which is widely damaging in commercial plantations, but the values vary for different geographical regions [1,5]
The current study explores the use of digital photography and leaf area index (LAI) to detect growth checks to banana plants using a replicated field trial design in Uganda imposed by challenge with a combination of R. similis, H. multicinctus and Meloidogyne spp
Summary
Establishing damage levels from biotic stresses and the resultant economic loss to crops requires optimisation between assuring the reliability of the information gained and the effort and resources invested in its acquisition. The plant parasitic nematodes that cause severe losses to global banana and plantain (Musa spp.) production provide a specific example of the difficulties of damage assessments. The yield losses they cause are a combination of reduced plant growth that extends the production cycle, reduced harvested banana bunch weight and toppling of plants with damaged root systems in storms [1,2]. Work in Costa Rica on the dessert banana cultivar Grand Naine ranked R. similis, P. coffeae, M. incognita and H. multicinctus in descending order of severity of root damage by equal densities of each nematode species but recorded M. incognita as causing the largest reduction in bunch weight [9]. Root necrosis may not be a reliable basis for assessing nematode losses to yield when Meloidogyne spp. are one of the economic species present [11] or generally when R. similis is not the only damaging species present
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