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ISHS Acta Horticulturae 166: Propagation of Ornamental Plants

FOSETYL-AL: CONTROL OF PLANT DISEASES UTILIZING THE PLANT'S OWN DEFENCE MECHANISMS.

Author:   D. D. I. Guest
Abstract:
Fostetyl-Al, which is the active ingredient in the commercially available disease-control chemical, Aliette, is able to control a range of Phytophthora and Pythium incited diseases in citrus, pineapples, avocado, ornamentals, vegetables, apples, strawberries, tobacco and grapevine. It also shows some activity against downy-mildew diseases, and, interestingly, some anthracnose, powdery mildew and virus-diseases, It is systemically active, and is able to move both up and downwards in the plant making it particularly suitable for the control of root and foliar diseases. It also shows curative activity if applied within 48h of infection.

Early tests showed that Fosetyl possesses very little antifungal activity in vitro, and this led to questions about its mode of action in the plant. At relatively high concentrations (500 μg m1-1 or more) some inhibition of fungal growth can be shown in vitro, and sporulation can be prevented at concentrations of 50 μg m1-1 or more. It is active in the plant at concentrations of 10 μg m1-1, however, which makes it unlikely that it is acting directly against the parasite.

One explanation may be that it is broken down in the plant to a more toxic derivative, which then acts on the pathogen. Fosetyl-Al does indeed break down in the plant to phosphorous acid, which can be shown to have some direct effects against the pathogen, but only under low-phosphate conditions. A number of other nutrients and growth conditions can affect its direct fungitoxicity, but the balance of evidence indicates that it is not a powerful enough toxicant to explain the activity of Fosetyl in the plant.

On the other hand, there are strong indications that Fosetyl is able to induce a resistant response in the plant following infection. Plants possess a number of natural defence mechanisms, and infection can only succeed if these defences fail. Plants expressing resistance can seal off the pathogen in dead cells which accumulate high levels of antibiotic compounds called phytoalexins. This response, called hypersensitivity, is one of the most common defence responses seen in plants, but it is by no means the only one.

Treatment of normally susceptible plants with Fosetyl results in the stimulation of these responses, much in the same way as resistant plants respond. Fosetyl induced protection is associated with a hypersensitive response and higher-than-normal levels of phytoalexin accumulation, and resembles the response seen in genetically-resistant plants. Chemicals which suppress the natural defence responses of the plant, such as enzyme inhibitors, render normally resistant and Fosetyl-treated susceptible plants susceptible in an identical fashion. Similarly, high temperatures

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