Guava (Psidium guajava L.) is a subtropical fruit and important in subtropical countries. These are round or oval and are eaten as a fresh fruit at two stages either at mature green stage or at fully ripe stage. Some varieties have only a few seeds, while others have a large cavity full of seeds. Cultivars vary greatly in sweetness and acidity. It can grow even in neglected soils. It is spoiled by many pathogens which include mainly fungi. Others are bacteria and algae. Some physiological disorders can also affect guava crop. The number of pathogens which affect the guava crop is 177 of which 167 are fungi, 3 are algae, 3 are nematodes, 3 are bacteria and 1 is epiphyte. Most of the fruit loss is caused by post-harvest fungal diseases. Fungi attacks the fruit and spoils the fruit during transit, storage and final transportation. These post-harvest diseases cause economic loss by reducing the quantity of fruit.
• Fruit canker
• Algal leaf and fruit spot
• Fruit root
It causes die back, twig blight, wither tip and fruit spots. Die back phase is caused by Gloeosporium psidii resulting in the death of plants. The plant begins to die backwards from the top of a branch.
Symptoms: Brown to dark brown colored acervuli are formed on the affected parts of the plant. Setae and conidia are formed in the acervuli. In moist weather, acervuli appear as black dots on twigs or fruits, which later produce pinkish spore mass. Spores are disseminated by wind or rain and initiate fresh infection.
Control: Although complete control is not possible, the application of 3:3:50 Bordeaux mixture and 0.22 or 0.33 per cent perenox give encouraging results in reducing the development of die back
Fruit canker caused by Pestalotia psidii.
Symptoms: The first evidence of infection on fruit is the appearance of minute, brown or rust colored, unbroken, circular, necrotic areas. The canker is confined to a very shallow area and does not penetrate deep into the flesh of the fruit. The infected fruits remain underdeveloped, become hard, malformed and mummified and drop in great numbers. Sometimes small rusty brown angular spots appear on the leaves. In winter the cankerous spots are cornmon but in rainy season minute red specks are formed.
Control: The spread of disease (in early stage of infection) is controlled by 3 to 4 spraying of 1 per cent Bordeaux mixture or lime Sulphur at 15days interval.
Leaf extract of Atadirachata indica and Ocimum sanctum inhibit the germination of spores. Use of O. sanctum extract is recommended, as it does not affect fruit flavor.
3.Algal leaf and fruit spot:
The alga Cephaleuros spp. causes spots on leaves and fruits and thus reduces the photosynthetic activity of the plant. The disease does not cause severe economic loss.
Symptoms: Cephaleuros spp. infects immature guava leaves during early spring flush. Minute, shallow brown lesions appear on leaves, and as the disease progress, the lesions enlarge. On immature fruits the lesions are nearly black. As fruits enlarge, lesions get sunken. Penetration of fruit is confined to several layers of cells beneath the epidermis. Fruit lesions are usually smaller than leaf spots. They are darkish green to brown or black in color.
Control: The control of alga can be achieved by spray of copper oxychloride (0.3%) 3-4 times at the interval of 15 days. Spray of copper oxychloride in rainy season is more effective. An ascomycetous parasite closely resembling Strigula astridiza on Cephaleurous parasiticus can be used for controlling disease biologically.
Phytophthora nicotiana cause fruit rot of guava. Rain and the wind are conducive for spread. The pathogen produces a great number of sporangia and spores on the surface of diseased tissues principally when the temperature is near 25°C and this is an important source of inoculum in the development of epidemics. Spores spread from the infected plant material or soil by rain splashes.
Symptoms: The disease starts at blossom and stem end or even anywhere on the surface of well-developed fruits. The skin of the fruit below the whitish cottony growth becomes a little soft, turns light brown to dark. Young, small and unripe fruits are more prone to infection. They become hard and woody. On leaves the disease starts as small brown spots at margin or tip, spread fast and affected the entire lamina.
Control: Soil solarisation. Destroy plant debris. Sprays of Bordeaux mixture, copper oxychloride (0.2%) and Dithane-Z-78 (0.2%) for control of guava fruit rot. Spray of Dithane-Z-78 (0.2%) at one-month interval.