Nematodes and other soilborne pathogens in agroforestry.

Abstract

What are the main factors that govern the build-up of soilborne pathogens? 2. Which strategies can be followed to avoid outbreaks of soilborne diseases? 3. How can agroforestry be a tool in the management of soilborne disease problems? 15.1. Introduction Soilborne organisms (such as plant parasitic nematodes, fungi, bacteria, phytoplasma, protozoa and viruses) are among the most underestimated of the factors which affect plant productivity in tropical regions. Because of their microscopic size and the non-specific symptoms of an infection, these organisms live out of sight and, generally, out of mind of the growers and plant protection workers. Root-knot nematodes are an exception in that they cause distinctive symptoms in the form of root galls, which are sometimes referred to as 'root elephantiasis' by subsistence farmers in central Kenya (Fig. 15.1a). Otherwise, most farmers and extension staff are not able to identify nematodes and other soilborne diseases (Sharma et al., 1997). Moreover, interactions commonly occur between nematodes and other soil pathogens, complicating any quick recognition of the problem and assessment of the damage done. Soilborne plant pathogens affect plants primarily through the infection of roots. These organisms occur as complexes in soils and in plant tissues, the nature of which are generally poorly understood and little quantified. In addition to pathogenic and parasitic organisms 1 , the soil contains a wide range of competitor saprobes, antagonists, beneficial organisms, yeasts, bacteria and nematodes (Fig. 15.2). The population size of each of these groups is determined by edaphic and environmental factors, as well as by the availability of host roots. Many vegetables, cocoa, citrus, tobacco Lupinus spp. Eucalyptus spp.; Pinus spp. P. americana ; Macadamia spp.; citrus Armillaria mellea (root rot) Coffee, tea, root and tuber crops C. cajan Acacia spp.; Erythrina spp.; Grevillea robusta Annona spp.; Macadamia spp.; Vitis spp.

Effect of Nutrition and Soil Function on Pathogens of Tropical Tree Crops

“…Little attention has been paid to the role of nutrition in alleviating diseases of tropical tree crops. However, for some diseases of tropical perennial species, a link is often observed between a deficient nutrient status caused by low soil fertility or poor plant nutrition and disease severity (Desaeger et al, 2004). Generally, plants stressed by various environmental limitations may be weakened and more vulnerable to disease and these include nutrientdeficient plants.…”

“…Pathogens that are mild in normal conditions of plant growth and exist mainly as saprophytes or endophytes, such as some Fusarium spp. and Alternaria spp., may cause severe disease under conditions of nutrient stress or aluminium toxicity (Desaeger et al, 2004).…”

“…Pest and pathogen attack of above ground parts of the plant may be encouraged by high N in the presence of low K and P (Desaeger et al, 2004); a high N/K ratio encourages insect herbivory by increasing the content of free amino acids in plant tissues (Marschner, 1995). A number of studies with cereals and other crops have shown that obligate pathogens in particular, such as Puccinia spp.…”

“…Root and collar rots are particularly severe in soils with low organic matter content, poor soil structure with a high level of compaction and poor drainage (Desaeger et al, 2004). If roots are restricted to the upper soil layers due to claypans or hardpans, this results in a greater exposure of roots to soil-borne pathogens.…”

“…If roots are restricted to the upper soil layers due to claypans or hardpans, this results in a greater exposure of roots to soil-borne pathogens. Similarly, old roots may leave pathogen inocula in cracks and fissures in the soil, increasing contact with new roots; an example of this is found in cases of Eucalyptus marginate and other native Australian species infected by the soil-borne pathogen Phytophthora cinnamomi (see Desaeger et al, 2004).…”