Post harvest diseases fruits and vegetables - Xavier University ...

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Postharvest Disease Summary 273 particularly prevalent after late-season rains, which play an important part in disease development. The fungi are common in soil and survive in their sexual state as thick-walled oospores. Fungi in the asexual state - sporangia and zoospores - are formed under moist conditions (Hough et al., 1980) and the mobile, swimming zoospores are splashed by rain or irrigation water onto low-hanging fruits in the tree. Rains and moisture can also carry the zoospores to higher fruits. Infection occurs only when the fruit remains wet for a relatively long period, during which the fungus may penetrate directly into the non-injured peel (Feld et al., 1979). If the fruit dries before the zoospores can germinate and penetrate into the peel, the infection process is arrested. Fruits infected on the tree will later drop onto the ground and, in fact, the appearance of infected fruits on the ground around the tree may indicate the presence of an active infection on the tree. At harvest, fruits which had unnoticed incipient infections on the tree are carried along with healthy fruits into the packinghouse. In such a case, the disease will develop during storage or transit. The decayed area on citrus fruit is firm and leathery, characterized by brownish peel discoloration. Under humid conditions, a white delicate mycelium usually develops on the infected area. Another characterization of the brown rot is its distinctive aromatic or fermented odor (Smoot et al., 1983). During storage, the disease can spread by contact with other fruits (Klotz and DeWolfe, 1961). At a later stage, secondary soft rot bacteria may invade fruits infected by brown rot and turn the firm decay into a soft one. 4. Colletotrichum gloeosporioides (Penz.) Sacc. [Perfect state: Glomerella cingulata (stonem.) Spauld. & v. Schrenk] Colletotrichum gloeosporioides, the causal fungus of anthracnose, is not included in the list of the major pathogens in citrus fruits. However, anthracnose may be a serious disease of tangerines, tangerine hybrids and other mandarin fruits harvested early in the fall, when long periods of ethylene exposure are required for promoting fruit color (Brown, G.E., 1975); it is usually only of minor importance on oranges, grapefruits and lemons. The fungus exhibits both the sexual state (perithecia with ascospores) and the asexual state (acervuli with conidia). Acervuli with conidia are formed in dead branches in the grove. Conidia from this source are spread by winds and rains and may infect citrus fruit on the tree only when it has been weakened by drought, frost damage or unfavorable http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE 274 Postharvest Diseases of Fruits and Vegetables cultural conditions. The infection may remain latent in the form of appressoria, in which case disease is manifested only during transport and storage (Fisher, 1970; Brown, G.E., 1975). Anthracnose may invade the fruits after harvest, in the packinghouse, in storage or in the market; it gains entrance to the fruit via mechanical injuries or dead buttons. Mature green fruits are sensitive to infection after prolonged exposure to ethylene during the degreening, since this treatment renders the intact peel easily penetrated by the pathogen. Affected fruits are characterized by the development of sunken lesions that, under humid conditions, form abundant salmon-pink masses of spores that later turn into brown-black masses. The incidence of anthracnose can be reduced by washing, which removes the quiescent appressoria (Smoot and Melvin, 1971; Brown, G.E., 1975), before degreening with ethylene. This, however, cannot be done commercially because washing retards the degreening process. Several factors affect the severity of anthracnose: (1) orange-colored fruits are more resistant to the disease than pale or green-colored ones (Brown, G.E. and Barmore, 1976); (2) degreening with ethylene at concentrations higher than those required for optimal chlorophyll degradation (5 |LI1 per liter of air) favors anthracnose development; (3) fungicide application prior to degreening reduces anthracnose development (Brown, G.E. and Barmore, 1976). Resistance to anthracnose was induced in mature green tangerines treated with ethylene after being washed to remove dormant appressoria (Brown, G.E. and Barmore, 1977). Application of ethephon (a chemical which produces ethylene on degradation) to Robinson tangerines 5-7 days before harvest significantly reduced the incidence of anthracnose (Barmore and Brown, G.E., 1978); control was attributed to the accumulation in the interior of the fruit of ethylene which induced physiological changes leading to the development of resistance. 5. Trichoderma viride Pers. ex S.F. Gray This fungus, which is responsible for Trichoderma rot in storage, is frequently considered to be a pathogen of minor importance. It can, however, cause serious loss in some citrus-growing areas. The fungus is known as a soil inhabitant and is capable of decomposing cellulose. Germinating conidia enter through mechanical injuries on the fruit, at the stem end or the stylar end (Gutter, 1961). This fungus is often overgrown by other organisms, particularly by Penicillium digitatum with which T. viride is often confused because of http://arab2000.forumpro.fr
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