The importance of tropical root-knot nematodes (Meloidogyne spp.) and factors affecting the utility of Pasteuria penetrans as a biocontrol agent

The conclusions of a collaborative study of the occurrence and importance of root-knot nematodes (RKN, Meloidogyne spp,) and of their control agent, Pasteuria penetrans, in parts of Europe, Africa, South America and the Caribbean are presented. Root-knot nematodes were estimated to reduce the yields of a wide range of horticultural crops by > 25% in Ecuador, Malawi and Tanzania, and by ca 10% in Trinidad and Tobago. The greatest proportion of infected crops were observed in Ecuador (205 of 207) and the least in Trinidad and Tobago (70 of 174). The mean gall index was greatest in Ecuador (5.5). Levels of galling were least in Senegal (1.6), even though 89% of crops were infested and virulent M. mayaguensis was widespread. In all countries. M. incognita and M. javanica were the most abundant species, but M. hispanica occurred widely in Burkina Faso, even in newly cultivated areas in the Sahile. Several new esterase phenotypes were detected, especially in Ecuador and Malawi. Juveniles (J2) collected from the soil during the surveys were examined for attached spores of P. penetrans. It was widespread (20 to 60% of RKN populations). except in Malawi and Tanzania < 10% were infected. and was found for the first time in Crete (Greece). Generally, < 50% of the J2 carried spores. The occurrence of fl penetrans was sometimes correlated with soil type e.g., in Senegal it was least frequent in sandy soils. Laboratory assays of the binding of spores of isolates of P. penetrans to populations of RKN indicated large variations in specificity and substantial interactions; differences between populations within a species of RKN were sometimes almost as great as those between species. In microplot trials in which an exotic isolate of P. penetrans was introduced (ca 10(3) spores per p soil). its incidence was not increased by increasing the frequency or intensity of the growing of RKN-susceptible crops. However. in two such trials st sites in Tanzania and Ecuador naturally infected with P. penetrans, there were large increases in the proportions of spore-encumbered J2 (up to 100% encumbered) and in the yields of spores (up to 3.3 x 10(6) spores per me dry root) in those plots amended with an exotic isolate. In these plots, numbers of J2 in the soil were decreased and damage by RKN was suppressed: gall indices were decreased (from > 8 to < 3) and yields were increased (by up to 30%). No such changes were observed in the unamended control plots. Increased suppression of RKN was also observed in a field trial, even in plots where RKN-susceptible and non-host crops were alternated. Increased suppression following amendment with the exotic isolate of P. penetrans was not observed at sites not previously infected with P. penetrans. Regression analysis of the results from the microplot and field trials indicated that tomato yields were decreased by > 5% for every increase of one in the gall index. Yields were increased by alternating tomato with leguminous crops in some trials, but not in others. It is proposed that, in natural infections, mutual selection produces a dynamic balance between the P. penetrans and the RKN whereby levels of infection are rarely suppressive. However, the introduction of an exotic isolate of P. penetrans, with a different attachment profile, can disturb this balance, resulting in a greatly increased proportion of infected J2 and females, increased yields of spores and more suppression of RKN populations.