Weather and Plant Parasitic Nematodes1

Abstract

Plant parasitic nematodes are minute and live concealed in soil pores or in plant tissues. Consequently, they are influenced by weather indirectly via soil or host plant. Surface tension forces determine whether nematodes can move over plant or soil surfaces and soil moisture suction forces as little as 100 to 200 cm water pressure (‐ 1 to ‐ 2 kPa) inhibit movement by thinning water films to less than 2 μm. However, nematodes can maintain their turgor up to pF 4.0 suction (10 atmospheres, ‐ 1 MPa), i.e. near the wilting point of plants when the relative humidity of the soil atmosphere is nevertheless 98%. Many species have dispersal stages that can withstand desiccation in the hostile aerial environment where relative humidities are far less. The activity of root ectoparasitic species is greatly affected by soil moisture and could possibly be modelled by cumulative rainfall. Soil temperature is the key factor in the development of root endoparasitic species. Despite the difficulties of assumed basal development temperatures and of average soil temperatures, development is effectively modelled by accumulated temperature. Complete rate/temperature curves are available for few nematodes. They can be described by relationships of the type: R = A + (B + CT)/(1 + DT + ET2) where R is the rate, T the temperature and A, B, C, D and E are constants. When fitted to data by the method of maximum likelihood, such curves more accurately portray temperature relationships than accumulated temperatures. Effects of weather on plant nematodes have been far less studied than effects on aerial pests and pathogens: much more work is needed.