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FALSE SPIDER MITE DAMAGE
FALSE SPIDER MITE DAMAGE-
Oranges, especially navels, have been afflicted with a necrotic spotting this winter. The spots are typically irregular to round and usually smaller than a dime. The lesions are dark brown and slightly depressed. Affected fruit are normally at or below waist level in the tree. Navels, Marrs and other oranges have been reported with damage, but grapefruit does not seem to have the problem.
The damage has been erroneously called leprosis, but is more appropriately referred to as exosis or nail-head rust, since leprosis is a disease.
The culprit has been identified as false spider mites by Dr. Vic French of the Citrus Center and confirmed by Dr. Carl Childers of the University of Florida. In fact, Dr. Childers came over from Florida last week to survey the situation in person and to present a seminar on Brevipalpus and some of the diseases associated with this mite. More on that later.
The outbreak seems to be concentrated in early oranges, especially in orchards which were not sprayed during late summer to early fall. Because of the lack of adequate water during the summer, many orchards were not sprayed since pest problems were mostly negligible. When it started to rain in September, many orchards could not be sprayed because of continued wet conditions into late October. Thus, false spider mites caused their damage.
Generally, we do not worry about false spider mites because they are not easy to scout and because the spray program for citrus rust mites pretty well keeps false spider mites in check. Thus, the season's weather did not favor rust mite development and control, so false spider mite populations did build to damaging levels.
NOTES FROM CHILDERS' SEMINAR-
There are three species of Brevipalpus in Florida and Texas-B. phoenicis, B. obovatus and B. californicus, all of which affect citrus and a large variety of other species. Females produce more females; males are not common. The life cycle is about 39 days at 70 , but only 21 days at 80. It thrives in high density plantings.
False spider mite injury to citrus leaves is an irregular to round, brown lesion surrounded by a yellow halo. The lesions can be on the margin or anywhere else on the leaf and very much resemble the symptoms we have been seeing on the fruit of oranges, except without the yellow halo.
False spider mites vector several virus and virus-like diseases of citrus and other crops. Perhaps most notable is leprosis which results in dieback, sparse foliation and heavy leaf and fruit loss. Leprosis nearly destroyed the Florida citrus industry in the early 1920's before sulfur became widely used for mite control. Leprosis is currently of major concern in Brazil, primarily on oranges. Childers opines that a major part of the reason for the relatively lower Brazilian orange production per acre is due to leprosis.
Another disease in Brazil which is transmitted by false spider mites is zonate chlorosis, which resembles leprosis on fruit and leaves, but rarely affects twigs. It is more common on tangerines and more common in coastal regions. A most distinguishing symptom of zonate chlorosis on leaves is the alternating elliptic zones or lines of green tissue and yellow tissue.
Brevipalpus has also been shown as the primary vector of coffee ring spot virus, green spot of passion fruit and leprosis of ligustrum-all in Brazil. Of most concern to U.S. citrus interests, however, is that citrus leprosis is common throughout South America and has recently been reported in Panama.
Fortunately, in all cases of Brevipalpus-transmitted disease, good mite control reduces or eliminates the disease problem.
Back to the false spider mite damage that we are seeing on oranges in Texas, Childers thinks the spotting results from false spider mite feeding injury that is subsequently invaded by secondary fungi that complete the damage.
And, if you want to see them, look at the outer margin of the necrotic spot-you'll likely find egg clusters and immature mites as well as mature false spider mites just beyond the boundary of the damaged area.
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Most growers should know how much water they will have to start the season, as irrigation districts will have made their allocations based on the water that is currently available. Naturally, any significant inflows into the reservoirs will be allocated as they occur.
Obviously, you also know that you can't bring a citrus crop to maturity with the water that is allocated at the present time-unless it is a very wet year, with sufficient rains coming just when you need them. Even growers who have long been conserving water with microsprayer or drip systems found out the hard way that when the district runs out of water, you can't even get all of the water that was allocated to you.
So, how do we get through 1999? With conservation, timely rainfall in the Valley and the watersheds, alternative water supplies and water transfer from other acreage, idle or otherwise. Surely 1999 cannot be as dry as long as it was in 1998, can it?
Once again, we have begun to see significant defoliation of the latest growth flush as the result of dry cold fronts and leaves that have not completely hardened. The situation is much worse in those orchards which had been harvested prior to the arrival of the recent dry frontal systems, while those still with fruit were not damaged so much. While that in itself suggests that soil moisture may be low, as citrus fruit does relinquish water to the leaves in time of need, soil moisture does not appear to be limiting, despite the scarcity of significant winter rains.
Basically, water demand during dry frontal systems exceeds the ability of the root system. Mature leaves simply shut down for the day by closing the stomates. Non-hardened leaves also close the stomates but they continue to lose considerable moisture because of poorly developed cutin (wax) layers on the leaf surfaces. With fruit on the tree, the fruit provides some water to make up the difference between demand and what the root system can supply, so firing is not so devastating as it is on fruitless trees.
Both grapefruit and oranges are affected, but oranges seem to be affected more than grapefruit. This may simply reflect the fact that considerably more orange orchards have been completely harvested.
CITRUS UTILIZATION TO DATE-
Despite a slow start of harvesting because of fall rains, the season is progressing very well. Total fresh shipments of all citrus is up 9.0 percent above the same time last year. Export grapefruit is 4.3 percent higher than a year ago, while domestic shipments are up 7.9 percent.
Early orange shipments, excluding navels, is a whopping 23.6 percent ahead of last year, but navel orange volume is down 7.1 percent. Obviously, navel production in the 1998-99 season is down from 1997-98, possibly as a direct consequence of the drought, since navel oranges are more sensitive to stress than round oranges or grapefruit.
As for quality of the crop, as measured by relative amount of harvested fruit that has been diverted to processing, grapefruit is a littler better than last season, with 19.1 percent of harvest having been processed as compared to 20.0 percent a year ago. Orange fruit quality is noticeably better, as only 17.7 percent of the orange harvest to date has been processed versus 28.7 percent a year ago at this time.
Some would argue that diversion to processing is not necessarily indicative of overall crop quality-and I agree. In some years of high production, grading standards can be tightened up to try to maintain better f.o.b. prices. Conversely, in years of lower production, grade standards are more nearly normal so as to maintain a higher supply of fresh fruit. Too, in some years, you just can't seem to sell Choice grade fruit at much, if any, above the cost of pick, pack and sell. In such situations with oranges, the returns for processed oranges may be higher than those for Choice grade fruit.
Nonetheless, calculation of the percentage of the harvest that is processed is the best means we have, from available data, to estimate overall crop quality. Other than some reliable means to periodically sample the packouts of each type of citrus, I know of no other alternative. Perhaps we could have a better idea if we could track the total volume of both Fancy (or Combination) and Choice fruit that is shipped-until that data is collected and made available, I'll just have to use the information that I do have.
Although about half of the grapefruit crop and a fourth of the orange crop, plus all Valencia production, still remains to be harvested as we move into February, it is time to get started on preparations for next season's production. Navel oranges should flush in late February, closely followed by other oranges, with the grapefruit flush coming in early to mid-March-all depending upon the weather, of course.
February is the traditional time to get started with fertilization and irrigation so that orchards are fully prepared for optimal spring flush, bloom and fruit set. Some growers like to get the jump on weed control in February, but only in blocks in which harvest has been completed. I don't think anybody wants to run herbicide booms under trees that still have fruit, as the number of fruit knocked off can be substantial. Too, herbicide application before the completion of harvest could delay final harvest until after the appropriate pre-harvest interval of the different herbicides used.
Still, there is plenty of time for herbicide application in March or April following harvest. Granted, the heavy rains of September and October may have diluted weed control efforts and delaying application until after harvest could result in control problems. Consequently, you may have to add a systemic contact material to the pre-emergent herbicides to regain good weed control.
In most cases, pest control efforts should not be required until April-more on that next month.
JULIAN W. SAULS, Ph.D.
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