Agros Citricos Grove Visit March 18-19, 2011 By Peter Spyke Introduction Peter Spyke and Dr. Pete Timmer visited the groves in March to inspect the potential of Greening (HLB) disease, and to determine the best approach for managing the groves. Dr. Timmer confirmed that the blotchy mottle symptoms appeared to be HLB. However, all PCR tests that have been run on Agros Citricos trees have come back negative no HLB present. Greasy Spot control and Dr. Pete Timmer and the Agros Citricos team in Pica Pica pruning remain as the two most significant loss of mature leaves on the trees, so any program related to HLB would have to fit with those programs. Dr. Timmer is a retired Plant Pathologist with the University of Florida who has worked extensively with citrus in the tropics. He is the authority on Greasy Spot, and has worked with HLB and other diseases as well. HLB Testing (Huang Long Bing (HLB) = Citrus Greening) Leaf symptoms that are consistent with HLB have now been found in Santa Rosa, Polencin, and Pica Pica. The blotchy mottle pattern is similar to the symptoms seen in other countries, and HLB has been confirmed in Yucatan, so there is still the strong suspicion that HLB is present. All samples sent for testing have been negative, including samples collected on this visit that included leaves with distinct blotchy mottle symptoms. Dr. Timmer agrees that the symptoms appear to be HLB, but while in Yucatan, Dr. Timmer contacted Mike Irey, who has set up and operates the HLB testing laboratory in Florida. Mike mentioned that another disease called Wood Pocket has been observed in Limes in Mexico, and that it can cause a blotchy symptom similar to HLB. Pictures on the internet show that Wood Pocket symptoms appear to be different than HLB and the symptoms on the Agros 1
Citricos trees, but it would seem to be prudent to consider the possibility that there may be another reason for these symptoms. Lemon tree showing typical growth pattern of HLB infection. The suspicion that it is present is based on more than just the blotchy mottle on leaves. Wood Pocket is caused by a phytoplasma, which is somewhat like a virus. HLB is caused by a bacterium, so it is possible to distinguish between the two with tests. Sending fresh plant material to Florida for testing would not be permissible, but Mike Irey suggested another strategy. It is possible to perform DNA extraction on a plant sample that would yield the DNA of the plant and any organism in the tissue. DNA samples can be transported, so if we can extract the DNA from tissue samples, Mike Irey can then test the DNA samples for both HLB and phytoplasma. Mike suggested a laboratory in Vera Cruz that could perform the DNA extraction. There would be no cost associated by either laboratory, so Agros Citricos is in the process of collecting samples and sending them to the Vera Cruz laboratory. It is hoped that with good DNA samples, Mike Irey can determine the cause of the symptoms seen in the trees. Foliar Nutritional Sprays for HLB In Florida, there are currently two approaches for dealing with HLB. If the disease is discovered early and the infection rate is low, growers can still use the strategy of removing infected trees and controlling the insect vector, Asian Citrus Psyllid, in an effort to reduce subsequent infestation in the grove. Where this is possible, it is still the preferred approach because an uninfected tree is always healthier and more productive than a tree infected with HLB. Growers have discovered, however, that if infected trees are treated periodically with foliar nutritional sprays, the progress of the disease can be slowed or even reversed to some extent. There is no consensus as to the best formula, so it appears that almost any 2
good nutritional formula has a beneficial effect. Three to four applications, applied when there are new leaf flushes, are sufficient to improve the appearance and performance of infected trees, provided that treatments begin before the trees have declined significantly. In Agros Citricos, if the symptoms observed do indeed turn out to be HLB, then the extent of the disease is beyond the point that exclusion by removal of infected trees and psyllid control is an option the infestation is too widespread. Therefore, on this trip one of the items of interest was the possibility of adopting a nutritional spray program. The logic was that if the trees are infected with HLB, nutritional sprays will make them more productive and extend their useful life. If not, nutritional sprays will have a beneficial effect anyway. It seemed to be a positive approach until the testing issues can be resolved. The questions were how to integrate a nutritional spray program with Greasy Spot control, and at an acceptable cost. Zinc and Manganese can help control Greasy Spot When discussing the possibility of using foliar sprays, Dr. Timmer told us about work he had done in the past that showed that Zinc and Iron have some activity on Greasy Spot, similar to Copper. While not as potent as Copper, they can be effective provided that the level of metals are sufficient. For these nutritional sprays, Dr. Timmer recommended a minimum of.6 kg/ha of metallic Zinc and/or Manganese to achieve control of Greasy Spot. Agros Citricos Foliar Nutritional Formula agua 1,000 l/ha Material Name kg/ha KNO3 nitrato de potasio 11.2 MgNO3 nitrato de magnesio 9.9 Urea urea 4.0 ZnSO4 sulfato de zinc 6.7 MnSO4 sulfato de magnesio 1.8 Cu (metallic) cobre 1.1 Boron boro Apply once each year Molybdenum molibdeno using commercial formula This creates options in the spray program to combine nutritional sprays with Greasy Spot applications without increasing the overall cost. We built a potential spray formula and sequence of applications using programs that have been successful in Florida for HLB and containing sufficient Zinc and Iron to provide some Greasy Spot control between sprays of Copper and Oil. In so doing, the number of sprays of Copper and Oil for Greasy Spot can be reduced by the number of nutritional sprays required to treat trees for HLB, which preserves the total number of sprays applied during the year. Then, by deducting the amount of nutrients applied in sprays from the amount of nutrients applied to the ground, there would not be an additional cost of materials. So, the resulting program is slightly more complex to manage due to more materials involved, but the cost is the same, or possibly less, than the Greasy Spot program currently used by Agros Citricos. 3
The preliminary nutritional formula for the new foliar sprays is shown above. This formula must be verified, because some materials may not be available, or the percentages may be different in Mexico. The Greasy Spot control relies primarily on Zinc since it is a more stable material, and is required in higher amounts by the trees. Greasy Spot Control Greasy Spot remains the highest priority in the Agros Citricos groves. Premature leaf loss caused by Greasy Spot infections weaken the trees by depleting carbohydrates and stored nitrogen. Fruit yields are reduced because the trees produce fewer flowers and fruit, and the fruit size is smaller. If the trees are indeed infected with HLB, the premature leaf loss will weaken those trees even more. Therefore, Greasy Spot control cannot be compromised, and is even more important if HLB is present. The current spray program relies on copper fungicides for Greasy Spot control. Copper is toxic to the Greasy Spot fungus, and can provide extended periods of control by affecting the hyaline mycelia that grow saprophytically on the underside of the leaves prior to the application (the kickback effect), and newly-germinated spores after the Effective Greasy Spot Control and timely pruning maintain trees in good condition due to lower amounts of premature leaf loss application (the residual effect). The combination of these two control methods is why the applications are timed to be about six weeks apart. Oil is also added to the sprays, currently about.5% by volume. The oil itself does not control Greasy Spot, but it does have other beneficial effects, such as suppression of mites that cause Negrilla and control of other insect pests. So, at this point, the Oil is an additive that supplements the copper fungicide, but is not active in the Greasy Spot control program. In higher concentrations, at least 1% by volume, oil can delay the expression of Greasy Spot symptoms in the leaves. Oil does not control the disease, but affects the progression of the disease if the oil concentration is high enough. 4
Since the goal of the program is to reduce premature leaf loss, higher rates of oil can be helpful, but again the oil does not reduce the amount of infection. In the case of Agros Citricos, the decision was to rely on copper, which will reduce Greasy Spot infection, rather than oil, as the primary tool. However, a single application of copper with 1% oil would have more beneficial effects than a spray of copper plus.5% oil due to the slowing of symptom development. Therefore, in the program, we have modified the copper-oil application Good Greasy Spot Control and timely pruning results in good production of fruit in the outer tree canopy, an by increasing the amount of oil in important part of achieving higher yields each spray. Since there are fewer sprays, the total amount of oil and copper per year is the same. The level of control will hopefully be better, however. The value of Greasy Spot control combined with timely hedging and topping is being reinforced in the groves. The pattern of tree growth is more favorable for formation of flowers and fruit where the Greasy Spot and Pruning programs have been successfully achieved. In other areas, due to delays in pruning and/or leaf loss due to Greasy Spot or poor drainage, the trees revert to very vigorous shoot growth, and lower fruit production. Leaf Density vs. Fruit Production Project Fernando has established comparison areas to determine the effect of leaf density on fruit production. In some trees, particularly in the shaded interior of the trees, there are very few leaves, but fruit is being produced on those branches. Fernando is curious about the relationship of leaves to fruit, so he has marked trees to monitor that either have fruit on branches with no leaves or fruit on branches that have retained their leaves. This is a very interesting question, since the leafless condition can be common with more vigorous trees that are shaded in the interior of the tree. The dense outer canopy both prevents sunlight from reaching the interior, plus it Fruit on shoots with no leaves good or bad? 5
interferes with penetration of Greasy Spot sprays, so the leaf density is low or nonexistent. The outcome of the experiment could influence choices about pruning or spray applications. Marking Shoots to Evaluate Greasy Spot Control: Dr. Timmer recommended a simple exercise to monitor the Greasy Spot control program by tagging flushes to determine when infection is occurring. To do so, records should be kept of the date of each major leaf flush, then a few shoots on each tree should be marked by tying flagging tape to them. These will become the monitored shoots. They should be on branches that will not be removed by hedging or topping. A suggestion would Marking shoots to observe effects of Greasy Spot as compared to spray dates and rainfall be to choose 4 shoots in different parts of a tree, and 5 trees per block, as a good starting point. To begin, the trees could be in Santa Rosa, and one block could be the older trees near the shop, and the other could be the bedded trees. So, 10 trees total, 4 shoots per tree. Each time a new flush emerges on the tagged shoots, the date of the flush should be recorded and the new flush tagged with a different color tape. As the season progresses, observations on Greasy Spot infection should be recorded. These can be examined and compared to spray dates and weather conditions. The goal is to learn more about the timing and materials used in the sprays. It s a good idea that should not require a big commitment. Alternation of Sprays can Improve Greasy Spot Control During the discussions during the visit, a strategy was developed to mitigate the impact of HLB while potentially improving Greasy Spot control, at no added cost. The basic approach will be to alternate nutritional spray applications with sprays of copper and oil, with the oil at an increased percentage in each spray. The total amount of oil will be the same as before, but since it will be applied in fewer sprays, the concentration in each spray can be higher. 6
In Florida, three or four nutritional sprays per year have been effective in reducing the impacts of HLB in infected trees. Agros Citricos has been applying six to seven spray applications per year the first intended to control mites and fungus diseases, and the remainder focused on Greasy Spot. With this program, four of the sprays will contain the nutritional mixture, and the remaining three will be copper and oil, applied in alternation. The details are included in the attached spray schedule. Timing of Sprays Citrus leaves are susceptible to Greasy Spot at any time, so the timing of the current spray program is based on the effects of copper rather than any specific tree growth events. With the nutritional sprays, timing will be more critical. Citrus leaves develop a wax coating that impedes penetration of water-based solutions. The coating increases in thickness as the leaves mature. Therefore, to achieve maximum uptake of the nutritional sprays, they should be applied as soon as the leaves have reach full expansion after a new leaf flush. At that point, the target (leaves) will be the largest size, and the wax coating will be thinner, allowing rapid intake of the nutrients. Applying the sprays to older leaves will work to some extent, but the efficiency is much lower. Therefore, instead of scheduling the sprays based on the calendar, the nutritionals will have to be applied at the optimum stage of tree growth. The copper sprays can then be applied at a certain point after a nutritional spray based on the calendar as before. The higher oil concentration in the copper sprays will help offset any risk of infection if the timing between sprays is not optimal for the fungicide performance. Fertilizer Program The fertilizer comparison in Reyes has shown that the normal fertilizer program and the double fertilizer program have resulted in much higher fruit production than no fertilizer. It has shown, however, that if fertilizer needs to be reduced for a short period, fruit production would be maintained at some level rather than collapsing. If fertilizer is withheld over a long period, however, fruit production would be much less than if fertilizer is supplied. No fertilizer area trees grow but do not produce as much fruit 7
Double fertilizer experiment. Trees more productive may be good to increase rates in healthy areas of groves Classic HLB blotchy mottle symptoms on leaves in Pica Pica, Polencin, and Santa Rosa At this point, the experiment has also proved that the normal fertilizer rates used in Agros Citricos are satisfactory. However, another interpretation is that in better parts of the groves, increased fertilizer can result in higher fruit production. Therefore, a topic for future discussion may be to vary fertilizer rates based on tree condition. A double rate would probably not be necessary the additional amount of fruit did not double in the experiment but a higher rate than normal may prove to be beneficial if the trees are very healthy in certain areas of the grove. It was decided that the value of the no fertilizer treatment has been established, so there is no further need to withhold fertilizer completely from that area of the experiment. Normal fertilizer can resume in the no fertilizer areas. Summary the performance of the trees regardless of the status of HLB. HLB is strongly suspected in the Agros Citricos groves, but has not been confirmed by testing. Tissue samples (leaves with blotchy mottle symptoms) will be collected and sent to Vera Cruz for DNA extraction. The DNA will be sent to Florida to be tested for HLB and other possible causes of the symptoms. In the meantime, Agros Citricos will modify the spray program to include nutritional sprays that have helped reduced the effects of HLB in Florida. The program is designed so that there will be no additional cost, and Greasy Spot control will be maintained. This should improve 8