PLANT PROTECTION 1 â Pests, Diseases and Weeds

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PLANT PROTECTION 1 – Pests, Diseases and Weeds EXAMPLES OF VIRUS & VIRUS-LIKE DISEASES Tomato spotted wilt Cause Tomato spotted wilt virus (TSWV), spotted or bronze wilt. TSWV occurs wherever its vectors occur. Losses can be serious and are likely to increase because the exotic Western flower thrips (WFT) is a very efficient vector of TSWV, has a very wide host range, readily develops resistance to insecticides and can reach very high numbers on host plants. There are different strains of the virus. Host range Has the widest host range of any plant virus. It can attack over 900 species of plant, including: Vegetables, eg tomato, potato, capsicum, lettuce, celery, eggplant, spinach. Ornamentals, eg aster, chrysanthemum, dahlia, Iceland poppy, nasturtium, petunia, zinnia. Fruit & nuts, eg peanut. Field crops, eg cowpea, lupin, tobacco. Weeds, eg dandelion, lamb's tongue, nightshade. Symptoms TSWV may be symptomless on some plants. On other hosts a variety of symptoms may be produced which are dependent on plant species, cultivar, growing environment and virus strain. Symptoms start to show 14-21 days after infection and may occur on leaves, stems and fruit. Plants may be distorted, stunted and show reduced vigour. In some cases leaves and/or whole plants may die. Vegetables. Tomato. Small areas of bronzing develop on the upper side of young leaves in the terminal growth and spread over the whole leaf (bronze wilt). Older leaves have bronze spots, rings or crescents up to 3 mm long between the veins. These spots may extend and join up. Affected leaves may wither and die and tissues blacken and shrivel until the shoots look as if they have been scorched by flame. Leaf stalks and stems may develop dark streaks. Young vigorous plants may be killed in a few days but in older plants disease may take several weeks to develop. Fruit on more mature plants may show irregular or circular blotches as they ripen (Fig. 141). Symptoms are usually obvious. Taste is not affected. Young fruits shrivel and fall. Broad bean. Tips of main shoots blacken and may die, dark streaks may develop on stems and black sunken lesions on pods. Capsicum. Leaves show yellowish parallel lines or concentric rings, the fruit is marked with yellow rings and blotches up to 10 mm across which may not show up until the fruit ripens. Rings and blotches may darken. Ornamentals. Chrysanthemum leaves are marked with irregular wavy lines, one inside the other. Leaves in very susceptible varieties go brown and die. Dahlia. Leaves develop yellow spots or rings. Later concentric yellow or brown rings or wavy lines appear. Symptoms are clearest on the first formed foliage, especially in early-planted dahlias. As the plant grows, new leaves formed during summer may only show slight mottling or no symptoms at all. Young stems may have brown to purplish streaks (Fig.143). Arum lily. Leaves develop yellow spots or streaks parallel to the veins (Fig. 144). The stunted and yellow appearance is distinctive. Nasturtium. Leaves develop straw-colored spots, become cupped, distorted, enlarged (Fig. 145). Diagnostics. Diagnosis of TSWV can be quite difficult. Symptoms are similar to, and can be confused with, nutritional disorders, pesticide injury, genetic patterns, etc, depending on the host (page 275, Table 54). Leaf symptoms usually occur initially on a few scattered patches of plants which gradually spread as thrips transmit TSWV to healthy adjacent plants. Knowledge of typical leaf symptoms on a specific host is required. Considerable experience is needed for a confident diagnosis. Fruit symptoms are usually easier to recognize. Plant tests. Confirm diagnosis with an on-site test, or send a plant sample to a diagnostic service (page xiv). Nepo viruses and their diagnosis www.daff.gov.au/ba/publications/nepoviruses ‘Overwintering’ In infected weeds, diseased stock plants, other host plants, eg volunteer crop plants, cuttings. TSWV is not seedborne, except for broad bean. PP1-4 TSW final Fig. 141. Tomato spotted wilt. Left: Circular blotching of tomato fruit. Right: Ringspots and other dark streaks on capsicum. PhotoCIT, Canberra (P.W.Unger). 286 Virus and virus-like diseases
. PLANT PROTECTION 1 – Pests, Diseases and Weeds WFT nymph WFT adult, feathery wings usually lie flat along the back Natural size about 1 mm long Thrips in dahlia flowers. Fig. 142. Various species of thrips transmit the tomato spotted wilt virus. Western flower thrips (WFT) (Frankliniella occidentalis) is the most efficient vector, it feeds on flowers, new leaves and buds and other plant parts. PhotoCIT, Canberra (P.W.Unger). Fig. 143. Tomato spotted wilt - symptoms on dahlia. Left and centre: Concentric yellow or brown rings or wavy lines on leaves. Right: Brown or purplish streaking on young dahlia stems. PhotoNSW Dept. of Industry and Investment. Fig. 144. Tomato spotted wilt symptoms. Yellow spots on leaves of arum lily. PhotoNSW Dept. of Industry and Investment. Fig. 145. Tomato spotted wilt symptoms. Irregular whitish blotches or green and yellow mosaic on leaves of nasturtium. PhotoNSW Dept. of Industry and Investment. Virus and virus-like diseases 287
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Insect secretions PLANT PROTECTION
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Control methods Cultural methods.
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Root structure of dock (Rumex spp.)
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PLANT PROTECTION 1 â Pests, Diseases and Weeds

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PLANT PROTECTION 1 â Pests, Diseases and Weeds