Diseases Affecting the Fruit - Part 1
Jose M. Amador
Extension plant pathologist
The Texas A&M University System
Diseases that primarily affect citrus fruit are usually of most concern to handlers and shippers. Many, however, begin in the orchard and can be reduced if proper measures are taken. These measures include avoiding picking fruit while it is wet from rain or dew and using lined boxes and trailers to minimize damage to the surface of the fruit.
Small lesions may be caused by fingernails, sharp objects and minor bruises. These openings on the surface of the fruit, although unseen, make it possible for microorganisms to gain entrance into the fruit. Sanitation in the field and packing house also is of the utmost importance. Infection can be decreased by preventing fruit from coming in contact with the soil and keeping harvest bags, field boxes, trailers and trucks' clean of debris. Most post-harvest diseases can be avoided if management and personnel are aware of the importance of keeping the fruit free of damage and contamination.
Many post-harvest diseases occur on citrus. Only a few of the most common diseases are discussed in this publication.
Of all citrus fruits, grapefruit is affected the most by melanose. Control of this disease plays an important role in the Valley, particularly in the production of gift fruit, because the disease greatly affects the appearance of the fruit. Small, brown, raised spots are found on the fruit. Usually, numerous spots coalesce into a big area, resulting in the phase of the disease known as "mudcake melanose." At other times, "tearstains" are found on the fruit as the result of spores being washed down in water from dew or rain. Infection and disease development occur only in the orchard when the fruit is small. Therefore, no spread or contamination of other fruit occurs during processing and storage.
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| Mudcake or sandpaper symptoms of melanose on grapefruit. | Tearstaining caused by melanose spores washed down the fruit by dew or rain. |
Although this disease is primarily a problem on leaves, the fungus also can infect the fruit. Infection occurs on cells surrounding pores, causing the fruit to become spotted or stippled. Areas surrounding these spots do not color properly because the cholorphyll present in the cells fails to disappear. If the spots are numerous, the fruit becomes unsightly, resulting in downgrading.
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| Closeup of greasy spot rind blotch of grapefruit. |
The disease derives its name from the small black specks formed on the rind in areas immediately surrounding the oil glands. The fungus that causes this disease prevents infected spots from attaining the typical yellow color of mature citrus fruit. The contrast between green and yellow color lowers fruit grade, even though there is no effect on fruit or juice quality. Thus, it is of little consequence to fruit used for processing.
No effective economic program has been developed for the control of flyspeck. Tests with fungicide sprays in the summer have increased the percentage of clean fruit in some cases.
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.
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| Chemical damage to fruit may resemble symptoms of parasitic diseases. |
Many chemicals are used in the production of citrus in South Texas. Herbicides, fungicides, insecticides and acaricides are needed to guarantee maximum production of marketable fruit. Although the proper application of these products will not result in damage to tree or fruit, sometimes mixing several products or applying these under certain atmospheric conditions will result in spotting, pitting or other damage to leaves and fruit. Because of the intensive agriculture practiced in the Valley, orchards often are located next to fields where other crops are grown. As a result, products that can be harmful to citrus, such as a cotton defoliant, sometimes are applied in close proximity. Care is needed in applying these products in the proper concentrations and avoiding weather conditions that could result in damage.
Acknowledgement
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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