ReviewEffect of temperature, pH, carbon and nitrogen ratios on the parasitic activity of Pochonia chlamydosporia on Meloidogyne incognita
Graphical abstract
Introduction
Root-knot nematodes (RKN) are a concern to many smallholders and commercial producers involved in intensive vegetable production in Eastern Africa (Gowen, 2002, Gowen, 2005). An estimate of 20% loss has been attributed to RKN in Kenya but losses may be up to 50% and total crop failure, principally due to Meloidogyne incognita and Meloidogyne javanica, can occur (Kanyagia, 1980). These nematodes can also exacerbate diseases, particularly vascular wilts (Gowen, 2002). Although not all smallholders will recognize nematodes as a biological constraint, up to 20% of vegetable producers use nematicides when growing tomatoes (Gowen, 2005, Oruko and Ndun’gu, 2001). Currently, many agricultural chemicals used for nematode control are no longer available because of health and environmental hazards associated with their use, besides being increasingly less effective and costly. These realities demand that nematode management should become an integrated program of practice, including alternative measures to the use of chemicals, for example, through the development of bio-management strategies and the use of selected biological control agents in combination with other control methods, in order to provide sustainable nematode control systems.
Most biological control agents require certain environmental conditions for optimum growth, infection or predacious activity (Sayre and Walter, 1991). Knowledge of the environmental conditions that affect the growth of a biological control agent is essential when determining its ability to control plant pathogens, as in the case of the fungus Pochonia chlamydosporia (Goddard) Zare & Gams used to control RKN (Sayre and Walter, 1991). Fungi require temperature levels that may differ from one isolate to another, for their growth and infectivity (Viaene et al., 2006). The optimum temperature for growth of the nematophagous fungus P. chlamydosporia is 25 °C, but this can vary and is not necessarily the optimum temperature for infection, depending on the isolate (Kerry et al., 1986). For example, the optimum temperatures for hyphal growth and parasitism of strain II of P. chlamydosporia is 25 °C and 12 °C, respectively (Irving and Kerry, 1986). In general, the survival of the fungus in the soil is limited by both high and low temperatures (Van Damme et al., 2005). Temperatures below 5 °C hinder growth of the fungus while little growth occurs at temperatures above 30 °C (Kerry, 2000).
In order to facilitate growth and multiplication of the fungus, nutrients from different sources are required. Locally available organic material, such as fresh crop residues or organic waste materials, can be added to the soil, a practice that has been known to reduce nematode populations. Pochonia is usually added to soil in a colonized rice substrate as an energy source for the fungus. However, in some fungi, high nitrogen/carbon levels can repress infection-related genes and may compromise parasitic ability (Ward et al., 2012).
Addition of organic amendments to the soil is known to increase the population of P. chlamydosporia but lowers infection potential due to the increased availability of nutrients to the fungus (Jaffee, 2002). The supply of carbon (C) and nitrogen (N) from organic amendments is one of the major factors responsible for fungal activity and growth (Segers, 1996). It is known that the potential of P. chlamydosporia to control nematodes depends on the level of C and N released into the soil after decomposition of plant-based materials (Chen and Dickson, 2004). Therefore, further research is required to consider whether addition of the fungus to soil already enriched by decomposed organic amendments enhances fungal growth. However, the fungus is limited in colonizing the soil due to its weak saprophytic nature, leading to suppression by other soil micro-organisms (Kerry, 2000).
Parasitic activity of P. chlamydosporia in the soil is challenged by many factors including chemical and physical aspects, pH being one of them, which can be a limiting factor on the growth and infectivity of RKN eggs. Variation in soil pH from acidic to alkaline decreased the infection of RKN eggs by P. chlamydosporia (Jaffee and Zasoski, 2001), the optimum pH for growth of P. chlamydosporia being pH 5 (Kerry et al., 1986). Little is known about the other factors affecting the parasitic activity of the fungus, including nutrition, particularly in soils rich in organic substrates. Therefore, the objective of this study was to determine the effect of soil temperature and pH, as well as levels of C and N, on the parasitic activity of P. chlamydosporia on M. incognita (Kofoid & White) Chitwood.
Section snippets
Effect of organic materials decomposed at different temperatures on growth and parasitic activity of P. chlamydosporia
An experiment was conducted using different organic amendments decomposed at different temperatures to assess the ability to support growth of P. chlamydosporia and infectivity on RKN eggs. M. incognita was multiplied on infected tomato cv. Tiny Tim plants (Solanum lycopersicum L.) grown in sterile soil at 25 °C under glasshouse conditions. Egg-masses were hand-picked from galled roots using fine forceps under a Wild M5 stereomicroscope (20×) six weeks after inoculation of the plants with
Effect of temperature on organic materials and on growth and parasitic activity of P. chlamydosporia
Decomposition of organic amendments at various temperatures significantly (P = 0.002, F-test) affected the number of CFU/g soil (Table 1A, Fig. 1A). Sawdust had a significantly (P < 0.05, LSD) higher number of CFU compared to the other organic amendments (Table 1A, Fig. 1A). Also, the number of fungal propagules in each decomposed organic amendment was significantly (P < 0.05, LSD) higher compared to non-decomposed material (Table 1A, Fig. 1A). The interaction of organic amendments and temperature
Discussion
The mode of action of organic materials added to soil to control soil-borne diseases and pests is not fully understood but may be due to: (i) release of toxic compounds from the plant material, (ii) release of toxic metabolites from increased microbial activity, (iii) enhanced growth of microbial antagonists and non-specific biological control agents, and (iv) general improvement of soil structure and fertility (Huang et al., 2006). This study has shown that the of population P. chlamydosporia
Conclusions
The results of this work can be used to improve the effectiveness of the fungus when applied in combination with the incorporation of non-costly, organic amendments which are locally available to Eastern African smallholders, as part of an integrated pest management approach to control RKN. The efficacy of P. chlamydosporia can be enhanced through addition of decomposed organic amendments whose carbon content is at least 20 times lower than that of nitrogen. Optimization of carbon and nitrogen
Acknowledgments
The authors would like to thank the Gatsby Charitable Foundation, United Kingdom for financial support through Rothamsted Research, UK. Penny Hirsch for her advice on the carbon:nitrogen experiment. In memoriam Prof Brian R. Kerry.
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