Managing Alternaria Brown Spot by L.W

Managing Alternaria brown spot By L.W. Timmer, R. F. Reis, S.N. Mondal and N.A. Peres anker and greening may be problem for production of Fortune tan - the diseases that are on gerine in Spain. More recently, we Fig. 1. Alternaria brown spot lesions everyone’s minds, but we have confirmed the presence of the on a Minneola tangelo leaf. Note the still have to deal with the disease in Peru, where it is severe on characteristic necrosis running up Cmany everyday problems of produc - Minneolas, and in Iran, where it the veins. tion. Alternaria brown spot is the most causes problems mostly for Fortune for a few days on the grove floor, but serious disease of many popular tan - and Page production. then ceases as the leaves decay. Spores gerines and tangerine hybrids such as Tangerine cultivars, including Min - can also be produced on fruit and Minneola and Orlando tangelos, Mur - neolas, Orlandos, Novas, Lees, twigs, but are relatively few compared cotts, Sunburst, Novas and others. Ponkan, Murcotts, Dancy and many to the production on leaves. However, This disease probably requires more others, that are susceptible to ABS lesions on fruit and twigs may be im - intense management than any of the carry a gene for susceptibility to the portant in the overw-inter survival of other fungal diseases. toxin. That gene is dominant and all of the pathogen. Alternaria brown spot (ABS) is the offspring of susceptible parents are In the 2004-05 season, Alternaria caused by the fungus Alternaria alter - also susceptible. Many cultivars devel - pressure was very low and growers nata . This fungus produces a toxin oped in Florida were derived from found it relatively easy to control the that is responsible for most if not all crosses of Dancy tangerine and thus disease in spite of quite favorable of the disease symptoms. ABS causes are susceptible to the disease. How - weather conditions for disease devel - black, necrotic spots on young leaves ever, within that group, cultivars may opment in spring and early summer. that can develop yellow halos as the vary in their tolerance of the disease. We suspect that the hurricanes in 2004 leaves mature. Sometimes the toxin is We have found that we can distinguish translocated up the veins and produces stripped many of the leaves and fruit the level of susceptibility of cultivars from trees, those tissues decayed a necrosis of the veinlets that is char - by inoculating leaves of different ages. quickly, and so there was little inocu - acteristic of the disease (Fig. 1). On For example, leaves of Minneola are lum available by spring, making ABS highly susceptible cultivars, entire susceptible even when they are fully easy to control. We doubt that we will areas of the leaf may be killed and the expanded and nearly mature, whereas be that fortunate again in 2006 and we leaves may abscise. Young twigs can only very young leaves of Novas are will have to be careful to manage the often be affected. At times, affected susceptible. Thus, we will be able to disease well. leaves drop and infected twigs die evaluate any new cultivars emerging With highly susceptible cultivars back at the tip, leaving twigs in the top from plant improvement programs like Minneolas, some growers have of the tree in the form of a shepherd’s more effectively. had good luck by planting trees on crook. Spots on fruit vary from small, The disease cycle of ABS is rela - very wide spacings. That facilitates black specks to lesions with large tively simple. Release of spores pro - rapid drying and reduces the disease corky eruptions (Fig. 2). duced on infected tissues is triggered pressure and also encourages better Sometimes, pieces of tissue dis - by rains or by sudden drops in relative fruit set. However, many fungicide ap - lodge from the le - humidity when the plications are still needed for good dis - sions, leaving dew dries. Those ease control. The Alter-Rater model depressed necrotic spores become air- (http://www.crec.ifas.ufl.edu/timmer/ areas on the fruit. borne and are carried Alterater.html) has proven effective in Fruit affected while by wind cur- Florida, worked well for control of the young often falls rents during the day, disease on Murcotts in Brazil, and has and production can then deposited on been used in Spain and Australia as be severely affected susceptible tissues, well. That system predicts the best by this disease. where they germi - time for sprays based on frequency of ABS is becoming nate and infect dur - rains, duration of leaf wetness, and av - much more wide - Fig. 2. Alternaria brown spot le - ing dew periods at erage daily temperature. Using a spread in the world sions on a Dancy tangerine fruit. night. Symptoms ap - threshold level of 50 points, 10 to 12 by unknown means. Lesions vary from small black pear in as little as sprays are recommended to achieve We confirmed its specks to large sunken craters. one or two days after good control for highly susceptible presence in Brazil infection. Spore pro - cultivars such as Minneolas. and Argentina in recent years and the duction begins only after lesions are Usually, a higher threshold and disease has become very serious in well developed about 10 days after fewer sprays can be used for Murcotts those countries. It is even a problem in they first appear and continues for in Florida. However, in Brazil, good drier climates such as Italy and Spain about 40 days thereafter. If leaves fall, control with Murcotts required 10 to and it has developed into a severe spore production continues on lesions 12 sprays. The weather did not appear CITRUS INDUSTRY • February 2006 to be very different from Florida since and after that time, sporulation on the Alter-Rater recommended a similar lesions dropped naturally even on number of sprays in Florida and São unsprayed leaves. Paulo, but the level of control achieved ADVICE FOR 2006 in Brazil was modest at best. ABS is so severe in some citrus-growing areas in The best advice for the 2006 season Brazil that it may not be possible to is to apply copper or strobilurin fungi - grow susceptible cultivars there. We are cides on a schedule determined by the not sure if the differences are due to the Alter-Rater model. Inoculum pressure isolates of Alternaria present there or may still be slightly less than in earlier the selection of Murcott grown. years, but a high level of control will EFFECTIVE FUNGICIDES be needed for good returns. The biggest problem will be to se - The most effective fungicides for Fig. 3. Spore production on leaf lesions lect an appropriate threshold for differ - of Minneola tangelo on leaves sprayed ABS control in Florida are copper ent locations and cultivars. For with Kocide, Headline or not sprayed. products and strobilurin fungicides cultivars that have a low value, a such as Abound, Gem and Headline. higher threshold can be selected, so We recommend that copper fungicides short residual and is thus not very ef - that sprays are applied only when con - form the basis of the program and that fective in disease control. We have ditions are most severe. strobilurins be substituted whenever found that both copper and strobilurins weather is hot and dry and there is a not only prevent infection, but also L. W. Timmer and S. N. Mondal are with danger of copper damage to fruit. Due greatly reduce spore production. Fer - the University of Florida, CREC, Lake Al - to the danger of development of resist - bam was ineffective in reducing sporu - fred; R. F. Reis is with São Paulo State University, Jaboticabal; N. A. Peres is with ance, strobilurins should not be ap - lation. A single application of a copper the University of Florida, GCREC, plied twice in a row. In Brazil, fungicide or Headline suppressed Wimauma. mancozeb is also used, but it has a sporulation for about 3 weeks (Fig. 3), CITRUS INDUSTRY • February 2006.

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