DESTRUCTION OF VIRUS-DISEASED PLANTS WITH A FIXED-DOSE PARAQUAT DISPENSER

Removal of the above-ground parts of infected plants only solves the problem in the current year, because the diseased offspring bulbs remain in the stock and will give rise to contagious plants in the next season. Removal of the bulbs is difficult under the currently used dense spacing and mechanical planting systems: digging inflicts great damage on neighbouring plants and thus causes unacceptably high yield reductions.

This problem led us to try to find a method to destroy the bulbs in situ by means of the herbicide paraquat. This compound kills the plant within 1–3 days and is also translocated to the bulb, where it gradually spreads throughout the tissues and ultimately kills the bulb completely.

In preliminary experiments, a few drops of paraquat applied in the axil of the lowest leaf gave promising results. Because bulb growers showed interest in the possibilities of this method, the following questions had to be answered:

• will all young developing bulbs be killed, or will some escape and act as virus sources in next year;
• what kind of applicator would give adequate protection to the user and the surrounding healthy plants?

To answer the first question, twelve bulb samples were collected from paraquat-treated plants. The daughter bulbs already present were stored. When examined some months later, many of the bulbs were found to be shrivelled or decayed. Of the total number of main bulbs (central "A"-bulbs) in all samples together, only about 7 % seemed to be unaffected. After planting of these main bulbs together with a number of apparently normal bulblets, only 6 plants emerged in the following spring, this number representing about 1.5 % of the total number of bulbs and bulblets in the samples. This result was comparable with earlier observations, which means that the application of paraquat eliminates a very high percentage of both the central bulbs and the surrounding bulblets, even those which still seem normal when harvested.

Various ways of applying the paraquat were investigated (Fig. 1). Toothpicks soaked in the compound and pushed into the plant stem were very effective, but gave rise to skin irritations, e.g. on the hands, of the operators. This mode of application is therefore prohibited now.

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