Storage Rots Diseases of Citrus
Green Mold ( Penicillium digitatum ) : The fungus penetrates the fruit rind through wounds. Symptoms begin as water-soaked area at the fruit surface followed by growth of colorless mycelium and sporulation (green color).
Blue Mold (Penicillium italicum) :The fungus penetrates through the uninjured peel and can spread from one fruit to adjacent fruit. Symptoms are similar to green mold except that the spores are blue.
Altenaria Rot (Alternaria citri ) : Fungus enters the fruit through their buttons. Preharvest treatment with gibberellic acid or postharvest treatment with 2,4 D delay senescence of the buttons and subsequent decay by Alternaria.
Control : Storage rots can be avoided by careful handling during harvesting to minimize cuts, scratches, and bruises. Treatment of fruits with Bavistin (1000 ppm), maintenance of optimum temperature range and relative humidity and exclusion of ethylene during transport can reduce post harvest losses.
Citrus Canker (Xanthomonas citri): It is the most serious bacterial disease of sour lime during rainy season. The disease symptoms appear on leaves, branches and fruit stalks. Canker lesions appear as yellowish spots, which gradually enlarge and appear as raised, rough brownish pustules. These pustules are surrounded by a characteristic yellow halo. Canker lesions on the fruits are confined to the rind only and do not penetrate into the flesh of the fruit. The market value of the canker-affected fruits is very much reduced.
Control : Pruning and burning all the canker-infected twigs before monsoon and disinfecting the cuts with Bordeaux paint can prevent the further spread of disease. Three sprays of Streptocycline 100 ppm (10 g of Streptocycline + 5 g Copper Sulphate in 100 litres water) or Blitox (0.3%) or neem cake suspension (1 kg in 20 litres water) during February, October and December can control the disease.
Tristeza Virus Disease: The aphid (Toxoptera citricida) transmits Tristeza virus disease. The symptoms begin with the dieback of small branches and twigs, yellowing of leaves and heavy bearing of small fruits. As the disease advances the symptoms, intensify resulting in severe chlorosis and mottling. The feeder roots of the affected plants die, the bark of the larger roots is distorted and brittle, and dry rot symptoms are observed in case of lateral roots. After 7-8 years the branches of the affected plant dry up completely and the plants wilts completely. Few trees show wilting symptoms overnight and completely dry up in 2 or 3 days. Hence, Tristeza is also called a quick decline disease.
Control : Good cultural practices, increasing the fertility levels of the soil and good drainage are useful in reducing the decline disease. The best method to check this disease is to control the aphid population by application of insecticides in the nursery and also in plantations.
Greening:This disease is spread through grafting and citrus psylla (Diaphorina citri). Affected trees are stunted with pronounced leaf and fruit drop. Some branches on affected tree exhibit severe twig dieback symptoms whereas the others are apparently normal. The fruits of affected trees remain mostly green even on maturity and the fruits, which are directly exposed to sun show a conspicuous yellow patch on the rind surface. Disease fruits are valueless owing to small size, distortion, low juice and insipid taste.
Control : As the disease is transmitted through grafting, the budwood used from the parent tree should be free from greening. Controlling the vector population can effectively check this disease in the field. Spraying Phosphamidon (0.025%) or Parathion (0.025%) controls nymphs and adults. Soil application of Dimethoate 10% granules around the plant basin gives effective control of citrus psyllids.
Citrus Exocortis: Exocortis is characterized by drying narrow strips of outer bark, which tend to separate from the inner live-bark. The outer bark slowly peels off as it withers. The affected trees show lack of vigour resulting in dwarfing.
Control : The disease can be prevented by the use of virus-free budwood. Grafting tools should be disinfected after pruning branches from diseased trees.