The objective of this work was to study the pathosystem tomato - Alternaria solani with regard to several new plant protection strategies. Agents and symbionts were tested alone and in combinations to find environmentally friendly but effective methods of early blight control. The included agents and symbionts were: (i) systemic acquired resistance, induced by ASM; (ii) plant restorative Spirulina platensis; (iii) growth-promoting fungus Piriformospora indica; (iv) symbiotic arbuscular mycorrhizal fungi; and (v) minimum quantities of the chemical fungicide Maneb. Interaction between ASM and Maneb, as well as between Piriformospora and Alternaria, were studied in vitro on artificial medium.
A new method to induce sporulation of Alternaria on axcenic medium has been developed, which is easily integrated into the normal process of cultivation as it requires only a switch of medium. Spirulina platensis, which is suspected to have plant restorative potential, has no diminishing activity on early blight disease. In tomato cultures, therefore, Spirulina platensis should not be used as plant restorative. Application of ASM via spraying reduced the susceptibility to early blight in some instances but increased it in others. This inconsistency may be explained by the duality of ASM toxicity, which harms on one hand the fungus (fungitoxicity) and on the other the host (phytotoxicity). In sprayed form both effects can occur in parallel with unpredictable outcome. A successfully established SAR, which is assumed to benefit necrotrophic pathogens as Alternaria may be a third contributing factor. The interaction between ASM and Maneb depends substantially on the particular Alternaria isolate as different isolates seem to have different levels of resistance against the two agents, resulting in synergistic, additive or antagonistic modes of interaction. Piriformospora indica has a measurable growth-promoting effect on tomato plants, which is different for the different tomato cultivars. Piriformospora seems to have - in addition to its growth-enhancing effect - protective potential against Alternaria infections. The mode of action of this ability to reduce the plant matter losses is yet unclear. Arbuscular mycorrhizal fungi are an effective bioprotectant against Alternaria in tomato. It has been demonstrated that mycorrhizal tomatoes have a significantly lower level of disease severity than non-mycorrhizal plants. The mechanism seems to be similar to ISR, which is also induced by associated organisms in the roots and effective against necrotrophic pathogens. It is well-known that P surplus hinders an optimal colonisation of host roots by AMF. Therefore, a restricted P supply is adventageous if the mycorrhizal protection method is exploited against Alternaria infection. Since mycorrhiza itself ensures a sufficient P availability for the host plant, no P deficiency will occur.
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