As the rainy season in Florida is set to begin, we caught up with Megan Dewdney to discuss citrus canker and other disease issues. Dewdney is a plant pathologist with the University of Florida Institute of Food and Agricultural Sciences.
Q: How has the prevalence of citrus canker been in Florida in the past year or so?
A: Dewdney: Citrus canker is common everywhere through the major citrus-growing areas. Most groves have the disease at least at a low level. We have been very fortunate in the last few seasons with dry springs. This has kept canker to lower levels in much of the state, but the disease can become bad locally if there is an isolated heavy rain and wind event when the fruit are young and approximately 0.25 inches in diameter for oranges and 0.5 inches for grapefruit.
Q: When should growers really start monitoring for canker in their groves?
A: Dewdney: Growers should have a sense of how much canker was in their groves from the previous season. It is a good practice to start scouting leading up to bloom for new canker on leaves. This is an indication of the inoculum for the current season. Continuing to monitor for the next two months is prudent to ensure that any outbreaks are caught early, particularly if there have been frequent rainstorms.
Q: When should growers initiate a control program for canker? Is there a threshold per se?
A: Dewdney: It is prudent to start a control program shortly after bloom, particularly if it has been wet during the spring. We have found that if the early-season infection is unchecked, then it is very difficult to bring the disease under control with later-season sprays. The dry weather this year is buying time for growers and will hopefully slow the disease because the leaves and fruit will be at a less susceptible stage by the time the rains start. There is not a particular threshold for canker. Once you start to see symptoms, it is difficult to get ahead of the disease. This is why fresh fruit producers are so careful about not letting canker get started in their groves.
Q: What is a good management program for canker?
A: Dewdney: Timing of applications is key. The timing will be based on average size of fruit rather than time of year. Oranges are most susceptible between 0.25 to 1.25 inches in diameter. For Valencia, applications after July are not necessary. Grapefruit are more susceptible, particularly from 0.5 to 1.5 inches in diameter, and for fresh fruit require treatment into October. We recommend a three-week interval for copper applications, but some growers prefer a shorter interval. Canker does not need as high a rate of copper as some of the fungal diseases. Usually, 1 pound per acre is sufficient and could be a little less, depending on the quality of the product, at the shorter intervals.
Q: Are there any new fungicides in the pipeline for citrus canker?
A: Dewdney: Recently, another plant defense stimulator, Aura Citrus, was registered for canker suppression on leaves. It has good efficacy on leaves and would be a good fit for protecting the leaves of smaller trees. Keeping the leaves protected reduces the amount of canker on fruit.
Q: A lot of growers are now using individual protective covers (IPCs), and some are growing in citrus under protective screen (CUPS). What effect do these have on canker incidence and spread? If growers have canker in IPCs or CUPS, what should they do?
A: Dewdney: We have found that canker is lessened by IPCs and CUPS. It appears to be a physical effect of slowing the wind-blown raindrops. We do see some canker under IPCs, but it is usually isolated to a small patch and minor. It is difficult to determine, but I suspect there are fewer stem lesions as well, which will lower the amount of continuing inoculum. If trees are in an IPC, they are usually small and vigorously growing leaves. If there was enough canker to become concerned, the recommendation would be to use a product like Actigard, which is a plant defense stimulator, because it is effective in suppressing leaf infections. Copper is not very effective for controlling leaf canker.
A similar effect is seen in CUPS, but on a larger scale. The wind speeds are not high, and raindrops are not particularly large. If canker makes an incursion, often from drips from the roof, it is normally easily contained. Growers could consider removing the affected flush and bagging it so as not to spread it down the row. I would spot-treat the affected trees and 10 to 20 trees around the area with copper. It is important to monitor for further disease in case it moves with regular maintenance tasks like spraying.
Q: What are the latest updates on citrus black spot?
A: Dewdney: Citrus black spot has been moving again. Last year, Manatee County was the newest county to have a find. This year, it was found in DeSoto County. Fortunately, there do not appear to be many affected blocks in either county. We still recommend fungicide applications starting in May at monthly intervals. It appears that the early-season applications are the most important. So, if growers are interested in just keeping fruit on the trees rather than trying to reduce the disease to a minimum, concentrate on the three early applications. With the dry weather this season, starting in May is sufficient. In wet springs, I recommend starting in April.
Q: Besides HLB, are there any other diseases that have been challenging for growers in recent years? What are the best management tips for those?
A: Dewdney: Algal spot seems to be getting more common around the state. This is the cause of the bright orange-red donuts or patches seen on branches. It can affect scaffold branches and twigs, with the potential to kill them. In my trials, I have not found any products that have worked better than phosphite fungicides. I recommend a 0.5 gallons per acre rate twice — in the beginning of May and the end of June.
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