Diseases Affecting the Fruit - Part 2
Jose M. Amador
Extension plant pathologist
The Texas A&M University System
Also known as oleocellosis, this disease has caused heavy losses to growers and shippers of Marrs and navel oranges picked early in the season with green rinds. It also may affect other citrus varieties. The spotting is caused by oil released from oil glands. The extruded oil kills rind cells, causing them to turn brown. The spots may vary from less than 0.5 inch in diameter to large, irregular areas involving much of the fruit's surface. The spots remain brown in contrast to the yellow color of the normal rind after degreening treatment. Fruits harvested wet are more seriously damaged by this disorder.
Fruit should be picked when the fruit surface is dry and should be handled carefully so that oil glands are not punctured or ruptured. Rind-oil spot can be prevented or reduced by: (1) picking fruit in afternoons of clear, sunny days, (2) deferring picking 2 or 3 days after a rain or an irrigation, (3) using fiberboard-lined field boxes or padded trailers and (4) having pickers use cotton gloves.
This fruit disease is caused by the same species of Phytophthora that causes foot rot. The fungus can attack fruit on the tree during periods of excessive rains or during irrigation. Infection by the fungus results in decayed areas that are brown, firm and leathery. At first, the fungus cannot be observed on the fruit. Later, a white velvety growth appears on the surface of the fruit, accompanied by a strong fermenting odor. Because the fungus is commonly found in the soil, fruit low on trees often is infected by rain-splashed soil. Winds can spread the actively growing fungus to fruit in the upper tree. The fruit must be wet for some time before infection by the brown rot fungus occurs.
The best control for brown rot is sanitation during transit and in the packinghouse. Disinfecting the boxes is important; treatment with disinfectant solutions and refrigeration are effective for prevention.
Stem End Decay
This disease is caused by the fungus Diplodia natalensis and often is detected at the packinghouse or in transit. Decay occurs around the stem end and advances in streaks down the side of the fruit. There is no fungal growth on the surface of the fruit.
Decay is reduced by dipping the fruit in fungicide solutions before placement in degreening rooms. Promptness in handling and shipping, as well as refrigeration during transit, help reduce losses caused by stem end decay.
Brown discoloration from stem end decay.
Green and Blue Molds
Green mold, the most important of these two post-harvest diseases, is caused by the fungus Penicillium digitatum. A rapid breakdown occurs in fruit punctured or bruised during harvesting and packing operations. The fungus enters the fruit through wounds. Therefore, the disease can occur on fruit on the tree, in the packinghouse, in transit, in storage and in the marketplace. A white mold is first seen growing on the peel. The mold later turns green because of the large number of green spores produced. Decayed fruit becomes soft and shrinks.
Green mold (left) and blue mold (right) cause soft decay of fruit, accompanied by heavy sporulation of casual organisms.
As its name implies, blue mold, caused by the fungus Penicillium italicum, differs from green mold by the bluer color of the powdery mass of spores that develops over the surface of the fruit. Conditions favoring the two diseases are similar. Both fungi grow best at a temperature of 75o F and their development is slowed by lower temperatures. Cooling citrus fruit during storage and shipping is an effective way to decrease damage.
Losses from green and blue molds also can be reduced by taking precautions at harvest and in transit not to injure the fruit and by treating the fruit in the packing shed with a fungicide solutions
Grapefruit Chilling Injury
Postharvest chilling injury of grapefruit.
Most citrus fruit can be injured when exposed to low temperatures for extended periods of time even though these temperatures remain well over the freezing point. Grapefruit is more susceptible than oranges to damage caused by chilling temperatures. The symptoms are varied, consisting usually of irregular, superficial discoloration of the surface of the fruit and rind pitting. The size of the sunken lesions tends to be smaller and lesions are more numerous as the temperature nears 32o F.
Chilling injury to citrus fruit can be avoided by carefully selecting temperatures for citrus storage or transport. However, because rot and decay increase as the temperature increases, a proper balance must be maintained. Fruit must be picked at proper maturity and handled carefully both in the field and in the packinghouse to avoid chilling injury.
Appreciation is expressed to Pete Timmer and Mike Davis, former plant pathologists, Texas A&I University Citrus Center, Weslaco, Texas, for photographs in this publication.
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