Demonstration of Cover Crops for Management of Plant-Parasitic Nematodes and Root-rotting Fungi in Hawaii Papaya
: Papaya is produced on 241 farms covering 4,055 acres in Hawaii, largely in small fields. Plant-parasitic nematodes (Rotylenchulus reniformis and Meloidogyne javanica) and root-rotting fungi (Phytophthoa palmivora, P. nicotianae, Pythium aphanidermatum and Calonectria crotalariae), the major soil pests of Hawaiian papayas, lower yields, reduce vigor, shorten crop longevity and cause the death of seedling transplants and established plants. The only registered soil treatment for papaya is metam-sodium which is applied before planting, is expensive and produces variable results. Cover crops suppress nematodes and soil fungi in pineapple fields so this project will determine if cover crops work equally well for papaya production. Cover crops are planted after the primary crop (papayas) is harvested for an inter-cycle period of two to three months. The cover crop is then incorporated into the soil and the field left fallow for another month before planting the new papaya transplants. Three cover crops will be tried: crotalaria (sunn hemp), marigold and rapeseed.
Project goals and objectives
1). To determine if nonhost cover crop plants for the nematodes, Rotylenchulus reniformis and Meloidogyne javanica, are hosts (non-, poor or good) for the fungi, Phytophthora palmivora, P. nicotianae, Pythium aphanidermatum and Calonectria crotalariae (greenhouse study).
2). To determine if incorporation of cover crop residue into soil reduces populations of nematode and fungal pests and enhances papaya survival and growth (greenhouse study).
3). To demonstrate the effectiveness of cover crops in reducing the status of nematode and fungal pests in a grower's field (field demonstration)
Host status experiments found that papaya and three cover crops had variable levels of susceptibility to two soil fungi and two nematodes. Based on greenhouse experiments, sunn hemp (Crotalaria juncea) showed the least reduction in shoot growth from both nematodes and Phytophthora palmivora, but was susceptible to Calonectria crotalariae. Papaya was extremely susceptible to P. palmivora, but not to C. crotalariae for the five isolates tested.
A greenhouse experiment to evaluate effects of incorporation of cover crop residues into soil on papaya growth showed that all cover crop treatments generally enhanced papaya growth in the absence of pathogens and in the presence of either the fungus (P. palmivora) or nematodes (Rotylenchulus reniformis and Meloidogyne javanica) alone as compared to no cover crop. Only sunn hemp stimulated papaya growth with both the fungus and nematodes present as compared to other treatments. Sunn hemp also enhanced shoot and root growth in the presence of nematodes more than any other treatment. Although cover crop residues increased papaya growth, they did not reduce nematode population densities in pots at harvest. Cover crop residues may enhance nutrients available for papaya growth and increase microflora density and diversity in the soil.
Cooperators also established a demonstration plot in a commercial field in Laie, Hawaii. Following production of papaya, four plots were planted with a sunn hemp inter-cycle crop that was incorporated after 10 weeks and four plots were left as bare fallow. Reniform nematode population densities in soil declined by approximately half between planting and incorporation in sunn hemp plots, but remained unchanged in bare plots. Cooperators will collect data on future papaya survival, growth and productivity and nematode densities, compile their results into a UH fact sheet and present the results at the annual Hawaii papaya industry conference.
Data not required at time of project