Interest in controlling weed plants using radio frequency or microwave energy has been growing in recent years because of the growing concerns about herbicide resistance and chemical residues in the environment. This paper reviews the prospects of using microwave energy to manage weeds. Microwave energy effectively kills weed plants and their seeds; however, most studies have focused on applying the microwave energy over a sizable area, which requires about ten times the energy that is embodied in conventional chemical treatments to achieve effective weed control. A closer analysis of the microwave heating phenomenon suggests that thermal runaway can reduce microwave weed treatment time by at least one order of magnitude. If thermal runaway can be induced in weed plants, the energy costs associated with microwave weed management would be comparable with chemical weed control.
The growing list of herbicide-resistant biotypes and environmental concerns about chemical use has prompted interest in alternative methods of managing weeds. This study explored the effect of microwave energy on paddy melon (Cucumis myriocarpus) plants, fruits, and seeds. Microwave treatment killed paddy melon plants and seeds. Stem rupture due to internal steam explosions often occurred after the first few seconds of microwave treatment when a small aperture antenna was used to apply the microwave energy. The half lethal microwave energy dose for plants was 145 J/cm 2 ; however, a dose of at least 422 J/cm 2 was needed to kill seeds. This study demonstrated that a strategic burst of intense microwave energy, focused onto the stem of the plant is as effective as applying microwave energy to the whole plant, but uses much less energy.
VARIOUS workers have reported a leaf spot disease of cultivated pelargoniums {Pdargomwn species), the earlier. European, records naming tiK fungal pathogen Macrosporium [>elargomi EU. & Ev. or M. macalpineanum Sacc. et Syd. Neegaard (1945) combined M. pelargcmi with Alternaria tenuis, and Munnecke (1956) has recorded A. tenuis as the causal agent of a leaf spot of pelargonium in the U.S.A. During 1%5. specimens of a regal pelargoniutn {P. X dome.sticutn Bailey) were received from Bidford-on-Avon. Warwickshire and of a zonal pelargonium (P. X hortorum Bailey) cultivar Master Crampel from a Lea Valley nursery, both lots showing leaf spot. In both cases A. tettuis was isolated from the spots. There are no published records of this fungus attacking Pdargottium spp. in this country but Atternaria tenuis, associated with leaf spot, was recorded in ninishire in 1963 by Dr. D. C. Gwynne. NAAS. Bangor. SYMPTOMS AND PATHOGENICITYZonal and regal pelargoniums were inoculated with conidial suspensions of ihe fungus and incubated in a high relative humidity.On the zonal pelargoniums, symptoms were observed five days after inoculation. Small water-soaked spots with a necrotic fleck in the centre, appeared on the undersides of the leaves. The lesions usually increased in size up to i in. diameter and were then sunken, necrotic and visible on the undersides of the leaves only (Plate IV. 2). Occasionally numbers of spots formed together producing larger irregular lesions. Tliere was no evidence of any further spread of the disease when tlie infected plants were kept in a glasshouse maintained at 15-21°C. Spread vvas observed on plants kept in the open during the summer months and A. tenuis was found sporulating on the lesions.Regal cultivars showed leaf spot symptoms most severely on the oldest leaves on which irregular brown lesions with reddish margins were conspicuous (Plate IV. 3). There was a tendency for coilescence of spots and some were concentrically zonated with the fungus sporulating on the lesion. On younger leaves necrotic brown areas were infrequent but " pin-head " lesions could be seen and these sometimes enlarged with leaf senescence.In further experiments marked leaves on plants of a number of different cultivars were inoculated and symptoms recorded five and 20 days after inoculation. The following cultivars were used:Pelargonium X hortorum-uonaX cuttivars. . Pelargoniutn X domestictjm-iegail cultivars. Duchess of Edinburgh. Hilda Jefford. Carnival. Bridesmaid.Pelargonium peltatum (L.) Ait.-ivy-leaved cultivars. Galilee and L'Elegante.Only the zonal cultivar Dr. McNanan and the ivy-leaved cultivars did not show leaf spot symptoms. Some zonal cultivars developed numerous pin-head
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