Biological Control of Microbial Plant Pathogens in Alternative Sources of Infection

Abstract

Microbial plant pathogens causing crop diseases are able to survive by infecting different weed species, self-sown volunteers and plants growing from asexually propagated infected tubers left in the field. In the case of plant viruses, the vectors form an important source of infection, especially for propagative viruses that are able to multiply in the vectors and pass on to the next generation. Biological control of pathogen inoculum present in alternative sources has been considered as a feasible approach to reduce the spread of the disease to the primary hosts, the crops. Fungal and bacterial biocontrol agents have been evaluated for their efficacy in suppressing the development of various weed plants that may serve as alternative sources of infection in the absence of primary host plants. Some of the fungal pathogens have been identified as mycoherbicides and they have been developed as commercial products for wider application. Entomopathogenic fungi effective against important groups of insects functioning as vectors of viruses have been identified. Extensive evaluation of their potential against plant virus vectors has to be undertaken under field conditions.

Microbial plant pathogens are able to survive in alternative sources which may be either plant species, in addition to the primary crop plants or insects or other organisms acting as vectors of plant pathogens. These additional sources may be present either in the same field or in other locations away from the field where the intended crops are grown. The plant sources may be other crops cultivated in the same farm or weeds that may allow the pathogens to multiply in the absence of crop plants acting as the reservoirs of pathogen inoculum. The vectors of plant pathogens may either mechanically carry the pathogens or they may permit multiplication of the pathogens as in the case of several plant viruses. Biological control of weeds and vectors employing biotic and/or abiotic agents, has been considered as an effective approach to reduce the quantum of inoculum of plant pathogens that can reach the crops, when they are planted in the ensuing season. As the weeds are known to be aggressive competitors for the nutrients applied to the fields, it can be expected that successful control of weeds would result in increase in the increased availability of nutrients to crop plants and consequent enhancement of yield with better quality. Likewise, biocontrol of vectors may reduce the inoculum potential and rate of spread of the diseases, leading to reduction of the negative effects of disease(s) and increase in yield levels. Investigations to examine the effectiveness of this approach are discussed in this chapter.