Apple (Malus domestica Borkh) is one of the popular and most favourite temperate fruit grown mainly in the countries of Europe, New Zealand, North America, Australia, China and Japan. Jammu and Kashmir is the leading state in area and production of apple. In Jammu and Kashmir, out of many diseases attacking apple plantation, various soil-borne diseases have recently emerged as a major hindrance in its productive cultivation under both orchard and nursery conditions.
Root rot in apple is mainly caused by Dematophora necatrix Hartig with its perfect stage as Rosellinia necatrix (Prill). This pathogen infects large number of hosts of about 158 species belonging to 45 different families, consisting of fruit and forest plants, herbs, Cereals, ornamentals, and other economically important plants. It affects many tropical (mango, avocado, coffee and citrus) and temperate (apple, pear, peach, almond, fig, kiwi, grape, olive, persimmon, sweet cherry) fruit trees, other field crops (beans and cotton).
It is a disease that attacks the roots of trees growing in wet or damp soil. These soggy conditions prevent roots from absorbing all the oxygen they require to live. As the oxygen-starved roots die and decay, their rot can spread to healthier roots, even if the soggy conditions have been rectified.
Weakened roots are more susceptible to soil fungus, which is another cause of root rot. The fungus may be present but dormant in the soil for a long time; when the soil becomes waterlogged, the spores can come to life and attack the roots, causing them to rot and die.
This fungal disease can survive in the soil for many years as spores. These spores are resistant to drought and to a lesser extent, chemicals. Fungal growth explodes with cool temperatures (around 56°F. or 13°C.) and ample rainfall. Hence, the highest incidence of fruit tree rot is during blossom time in April and during dormancy onset in September.
The most important microscopic feature of this fungus is the presence of pear-shaped swellings in the hypha, located near the septum, which has been widely used to identify the species. Such pyriform swellings in superficial mycelium are greater than those in submerged mycelium (Khan, 1959).
Symptoms of the disease on the small roots can be detected during the early stages of the infection before the pathogen rots the rootlets. Infected rootlets tend to be rotted and surrounded by a thin white mycelium coating. Diseased trees are easily uprooted in advanced infection phases because all small and medium roots are lost and the root system is very much reduced. Infected roots are usually covered by white cottony mycelium and mycelium strands colored white or dark brown to black that often extend under the bark and into the surrounding soil. The fungus colonizes the bark and external parts of wood at the base of trunk and on large roots. The bark which rots appears sunken and on infected tissues at the base of the plants which are dead, the pathogen most usually forms brown mycelial masses and sclerotia, from which synemata containing conidia grow. The roots are fully devoured in advanced stage (no fibrous root system) and results in, and diseased seedlings or trees are easily uprooted from the soil.
The affected trees display general declining symptoms which are not differentiable from those of other root rot pathogens like Armillaria mellea and Phytophthora spp. The type of tree species and the environmental conditions present can lead to a rapid decline of plants which quickly die (apoplexy) or show continuous weakening of trees, and can remain alive for some years. Apoplexy is very common for nursery seedlings and for new orchard plantations. The infected trees die within a few days after appearance of initial symptoms. The trees generally develop an unthrifty appearance in case of progressive weakening. Fruit size become small and shriveled, while as leaves show in-curved margin, change in color (yellowing and/or reddening), small in size and premature fall. In trees which are diseased, there is absence of new shoots and root growth. Whenever moisture and temperature conditions are unfavorable for growth of the tree, these symptoms worsen year after year.
To sum up, the symptoms of root rot are easier to spot above ground.
The most accurate way to diagnose this decaying disease is to dig below ground to see if decaying is taking place.
Root Rot pathogen can survive a significant amount of time as mycelium on its various hosts. Apart from those of commercial significant crops like Apple, the pathogen also survive on variety of weeds which promoting its spread in standing fruit trees and its survival upon clearance of contaminated orchards. It can also survive as saprophyte in the soil on dead roots and other plant debris rich in cellulose. It also can survive in the soil as high-quantity of microsclerotia.
Many factors in soil help in its survival. It spread is easily loosely packed soil (high sand content and average water quantity). The growth of pathogen was significant at maximum water holding capacity of soil and reduces rapidly with less moisture content, being zero at wilting point. Growth of mycelium is best at 22.5 to 25.5°C, optimum temperature being at 24 °C. The growth in the dark is higher, since the light has a strong inhibition effect. Soil pH exerts little effect, as pathogen can grow well at a pH of 5 to 8 and continues to grow even at pH 4 and 9. In the fresh soil the mycelium grew sparsely at first, in 5 days spreading to around 10 mm. There was, however, no additional development over the next 7 days, and the mycelium disappeared after 27 days.
Management of root rot is challenging by its ubiquitous presence in the soil and by the number of different attributes such as, drought resistance, tolerance to a broad range of soil pH, wide host range, deep penetration into the soil and the ineffectiveness of various common fungicides.
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