Diseases and Pests of Herbs: Lean about 1. Keeping Bugs away from Herbs 2. How to get rid of bugs from Herbs
Diseases and Pests of Amorphophallus:
A severe foot rot, caused by Fusarium moniliforme Sheld., has been reported from Allahabad during the first two months after planting. The rot starts as a water-soaked lesion at the collar region. The affected plants show slight pale colour at the tip of young leaves, which gradually spreads to the leaf blade and sheath. Thiram and Benlate (500 ppm) are effective in controlling the pathogen. Alternaria amorphophalli Vasant Rao is reported to cause leaf blight in the later stages of the crop. It can be controlled by spraying one per cent Bordeaux mixture.
Sclerotium rolfsii Sacc. produces a corm rot. A mosaic disease on the corm is reported to be caused by an aphid-borne virus. It causes proliferation of the buds in the corm imparting to them a distorted shape as compared to the healthy round corms. Selection of disease-free sets for planting eliminates the disease in the next season. There is no serious pest affecting the plant. Occasionally, termites or white ants attack the planted seed corms. Mixing crude oil emulsion in irrigation water prevents the spread of this pest.
Diseases and Pests of Atropa Belladonna:
The belladonna crop is subject to infection by several fungi. Fusarium solani and Pythium butleri Subram. cause root rot, resulting in sudden wilting. The stem of the affected plants darkens and shrivels at ground level, and the roots become discoloured and soft and emit an unpleasant fermented odour.
They wilt and die suddenly, mortality being upto 25 per cent when the temperatures are high. Planting the seedlings in sterilized soil reduces the incidence of the disease. Chloropicrin spray is recommended to control the root rot. Ascochyta atropae Bres and Alternaria tenuissima.
The young crop is infested by cut-worm (Agrotis ypsilon Rottl. and A. flammatra Schiff.) and white grubs; the cut-worms are most virulent during the dry months of June and July. Early transplanting during April-May helps to reduce the incidence of the pest. Application of five per cent Aldrin dust at 20-25 g/sqm of nursery beds before sowing protect the crop. Drenching of seedlings or transplants with a solution of Chlorodan reduces the pests in the field.
The common potato beetle is prevalent on the crop during summer. Sprays or dusts containing Rotenone (0.5-0.75%) are effective in reducing the damage.
The larvae of Gonocephalum and Lagycus spp. infest the roots and leaves of belladonna. The larvae of Heliothis armigera bore the aerial parts causing wilting and death. Flea beetles often feed on the leaves but seldom kill the plant. Tobacco worms cause damage occasionally. Besides these, a number of other caterpillars and beetles are reported to attack the leaves, flowers and the fruits.
Diseases and Pests of Chilli:
Chilli is attacked by several fungi, bacteria and viruses.
Die-Back:
It is caused by Colletotrichum capsici (Syd.) Butler & Bisby and is a serious disease of chillies; circular sunken spots are formed on green or ripening fruits, favoured by moist weather and shade, and heavy dew on the leaves. Infection mainly starts from flowers, or through the petiole. It gradually spreads downwards; branches dry up, followed by flower drop, and blackening and shrivelling of fruits or fruit rot.
Since the lower portions do not generally suffer, removal of the diseased portion up to the healthy parts and burning them reduce the disease. Since the disease is seed-borne, it can effectively be controlled by treating the seeds with Ceresan (0.2%) or Thiram (0.2%) followed by three sprays of Difolatan (0.2%) starting 60 days after transplanting.
Spraying Bordeaux mixture (1%; 563 l/ha) just before flowering and after fruiting is useful in controlling the disease. Brassicol, Bisdithane, Ceresan, Copper Sandoz and Fytolon also reduce the disease. Mancozeb (2.5 g/l of water) at an interval of 15-20 days can be sprayed for checking dieback and fruit-rot. Foltat (0.2%) is also reported to be an effective compound. Alternaria sp. also is reported to cause dieback in Allahabad apart from leading to formation of black spots in concentric rings on the leaves.
Anthracnose:
It is a serious disease of chilli caused by Gloeosporium spp., Glomerella cingulata (Stonem.) Spauld & Schrenk and Colletotrichum capsici (Syd.) Butler & Bisby. The disease occurs in many parts of India; the plant is affected at any stage, mostly during the monsoon. Anthracnose causes grey or yellow spots and patches of decay appear on the young and half-ripe fruits. They dry and shrivel and drop prematurely. Sometimes, reddish brown spots are also seen on the seed coat.
The organism enters the seed-cavity and infects the seeds. Treating the seed with organomercuric compounds and with Thiram or Captan or Agrosan GN (2.5 g/kg seed) can control the disease; spraying Dithane M-45 or copper oxychloride or Zineb at an interval of 10-15 days also controls the disease; Benlate and Brestan, Thiabendazole and Demosan are found to be helpful at various stages of attack. Removing and destroying the infected plants will have added advantage.
Pests:
The major pests that cause severe damage to the crop are sucking insects, such as aphids (Aphis gossypii) and thrips (Scirtothrips dorsalis). In the early stages, these pests cause complete crinkling of leaves and stunting. In addition to these pests, white ants also attack both the species of chillies but the damage is localized. Generally, Methamidophos and Acephate are recommended as the most effective compounds against chilli-pest complex.
Aphids:
The cotton-aphid (Aphis gossypii) and green peach-aphid (Myzus persicae) are common pests. The colonies infest the undersurface of leaves and tender growth including flower buds; these mostly establish from the stage of flowering. Both nymphs and adults suck the sap and the honeydew drop excreted on the lower leaves, and these show the development of sooty mould at a later stage, resulting in yellowing and dropping of the tender portions.
Five foliar sprays of Acephate (0.1%), methyl demeton (Metasystox) (0.5%) and Monocrotophos (Nuvacron) (0.1 %) when applied at 10-day intervals give maximum protection against aphids. Malathion 50-EC, Nuvan (0.05%) are also effective foliar sprays to control the aphids. Pirimicarb (0.1%), Lannate (90% WP), Fenvalerate (0.01%) and Heptachlor have also been found to control the aphid. Ethiofencarb (0.05%) and Methamidophos (either Tamaron 0.06% or Monitor 0.05%) are equally effective.
Application of Aldicarb, Phorate and Carbofuran to soil, though effective, may have toxic residues on the green fruits. Combined spray of urea (2%) with Methamidophos (0.06%) gave 99 per cent reduction in population of M. persicae at Madurai in Tamil Nadu.
Diseases and Insect Pests of Cinchona Succirubra:
Cinchona has been affected by a number of fungal diseases in all the countries wherever attempts have been made to cultivate the tree. Most important diseases which cause significant damage are damping-off cinchona seedlings resulting into rotting of young plants. The causal organism is Rhizoctonia solani. Treatment of seeds with any organic mercurial dust has been found useful in reducing the damage.
Tip blight disease caused by Phytophthora parasitica is characterised by necrosis and death of terminal branch and leaves of young seedlings as well as trees. The same fungus causes girdle canker in older trees. Destruction of dead plants and spraying of Bordeaux mixture has been found useful in controlling the disease.
Root rot, the most destructive disease, is caused by Phytophthora cinnamomi which causes root rot in young seedlings and later on also causes canker formation in older trees. Destruction of dead plants and spraying with any prophylactic fungicide is useful.
Collar rot is caused by Sclerotium rolfsii resulting into rotting of the collar region in the young trees. Destruction of the dead plants and earthing up of the trees help in reducing spread of the disease.
Some of the other fungal diseases are root rot caused by Armillaria mellea, root rot caused by Pythium vexans and leaf spot caused by Rosellinia sp.
Diseases and Pests of Hibiscus Esculentus:
H. esculentus is subject to dry root rot [Macrophomina phaseoli (Maubl.) Ashby], leaf spot (Cercospora hibisci Tracy & Earle), anthracnose (Colletotrichum hibisci Poll.), mildew (Erysiphe cichoracearum DC.), seedling blight (Phytophthora palmivora Butler) and fruit rot (Pythium indicum Balakrishnan). The plant is also affected by yellow vein mosaic, a virus disease common throughout India.
The characteristic symptoms of the disease are- clearing of veinlets followed by chlorosis of veins, vein swelling, slight downward curling of leaf margins and twisting of petioles, and general retardation of growth. The white-fly, Bemisia tabaci Gen., is the insect vector of the virus. The following control measures have been suggested- eradication of H. tetraphyllus, the wild host of the virus; observing a closed season of two months during summer between two successive crops; roguing diseased plants at the earliest stage of infection; spraying the crop once in 3 weeks with fish oil rosin soap; and keeping fields clean of weeds.
A top dressing of Nicifos is reported to check the disease. Spraying of plants with Pyrocolloid (1 part in 800 parts of water) or Derriphyton (1% soln) destroys white flies and thus, reduces the spread of the disease. Sprinkling of plants with wood ashes is also reported to reduce the spread of the disease.
Almost all the insect pests of cotton are known to attack bhindi. Cotton jassids suck the sap from the plants and crumple the leaves; spraying with 0.2% DDT (wettable powder) is employed as a control measure. Cotton bollworms bore through the growing shoots and young fruits; affected shoots and fruits are handpicked and destroyed, and the land is kept in a fine mulch. Red mite attacks the leaves and sucks the sap; the pest is controlled by dusting with sulphur or with a mixture of sulphur and lime (1:5); treatment with 15-25% DDT emulsifiable concentrate is also effective.
Diseases and Insect Pests of Ginseng:
Perennial erect herb, 30-45 cm high; root fusiform, 13-16 cm long, 5 cm diam., often branched. Leaves palmatisect, long petiolate; leaflets usually 5, sometimes 6-7, petiolated, thin, obovate-oblong, serrate. Inflorescence an umbel; flowers consist of 5 sepals; petals 5, small, ovate, white or yellow-green; stamens 5, anthers small, heart shaped; styles usually 2, sometimes 3, curved, persistent; ovary rounded, bright crimson when ripe, 2 seeded.
There are a number of diseases which cause serious problems in ginseng plantation. The most important diseases are Alternaria blight, Phytophthora blight, anthracnose, grey mold, and Rhizoctonia stem rot.
Alternaria blight is caused by Alternaria panax. The symptoms appear as dark brown spots in the stem just above the ground level followed by appearance of dark brown spots on leaves and flowers often causing death of plants. The disease can be controlled by regular spraying of Dithane Z-45, Maneb and Bordeaux mixture.
Phytophthora blight is caused by the fungus Phytophthora cactorum. The disease is characterized by appearance of water soaked patches on the stem and roots followed by rotting. The disease can be controlled by application of Metalaxyl.
Anthracnose is mostly serious in Korea and is caused by Colletotrichum panacicola. The symptoms consist of appearance of brownish black lesions dotted with black bodies of the fungus on leaves and stem. The disease can be controlled by spraying of Mancozeb, Polyoxin or Iprodione.
Grey mold caused by Botrytis cineria and can be controlled by application of Benomyl.
Rhizoctonia stem rot is caused by Rhizoctonia solani which causes appearance of black lesion on the stem near the ground level. The disease can be reduced by application of Rizolex. Other diseases reported from Korea and USA are root rot complex (Fusarium solani), Phoma blight (Phomapanacis), Ramularia blight (Ramularia sp.), and root knot (Meloidogyne incognita).
Some of the important insects reported from Korea are black cut worm (Agrotis ipsilon), common cut worm (Agrotis fucosa), dark grey cut worm (Agrotis tokionis), Japanese wheat wire worm (Agriotes sericeus), Korean black chaffer (Holotrichia diomphalia), and large black chaffer (Holotricha morosa). All these can be kept in check by regular spraying of Diazinon.