Sighting in on a target spot disease management plan

Post-harvest foliage management is important for blueberries produced in both deciduous and evergreen systems in Florida.  Healthy leaves maintained through flower bud differentiation in fall will ensure sufficient carbohydrate (sugar) reserves for uniform, early, berry production and leafing the following spring in deciduous production.  In evergreen systems, foliar diseases that carry over on leaves reduce the vigor of plants, but also can produce spores that infect fruit.  Anthracnose and rust pathogens infect both leaves and fruit and can lead to post harvest fruit rot (ripe rot) and fruit quality issues (rust infection of fruit).  Some varieties are more susceptible to summer foliar diseases than others.  Jewel is an older variety still widely grown that suffers from several foliar fungal diseases during the hot humid Florida summers.

It was on Jewel in 2014 that a new disease was discovered among the anthracnose and rust lesions causing premature defoliation.  The disease was target spot caused by <em>Corynespora cassiicola</em>. The pathogen is a fungus that is not new to Florida, and target spot diseases can be serious issues on both commercial tomato and squash production, but this was the first time it had been found in North America on any blueberry.  In Argentina a few years earlier in 2006, the disease was reported to have caused severe early defoliation on O’Neal southern highbush blueberry and that was the first report on a <em>Vaccinium</em> species.  It’s not clear why the disease hadn’t been reported before 2014 in Florida, given the pathogen has been around and is known to cause disease on a wide range of plants.  Unfortunately, there’s a similar story in what’s going on with bacterial wilt and blueberries in Florida.

In the last two seasons, growers have reported severe defoliation from target spot on several varieties (Arcadia, Biloxi, Endura, Jewel, Kestrel, Misty) and there are more not listed that are susceptible.  It can be difficult to differentiate the disease from anthracnose, and often both diseases will occur on susceptible varieties like Jewel at the same time.  Symptoms of target spot tend to be in smaller lesions than anthracnose, partly because the leaves with target spot lesions seem to fall from the plant sooner than they do with anthracnose.  In other words, they fall off before target spots get too big.  The lesions are reddish brown and have round to irregular edges.  As the lesion expands, the color can vary in concentric rings resulting in a “target” or bullseye sort of pattern.

The lesions produce many spores during periods of hot wet weather, and those spores, like anthracnose pathogen, can be spread by rain or irrigation splash.  Overhead irrigation provides extra water in the canopy as well and can make foliar disease more severe when used.  Spores are not produced in structures like they are with anthracnose, so you may not see the tiny black dots in the lesions.  As lesions age though, other opportunistic fungi can start to colonize the centers that do produce black fruiting bodies, so keep that in mind.  Submitting a sample to a disease clinic is the best way to know for sure.

Fungicides used for rust and anthracnose should also provide control of target spot, but some growers have reported that target spot is much harder to manage.  A very few isolates in 2017 from one such farm were collected and tested for sensitivity to Abound, and all tested sensitive, so it does not appear that fungicide resistance is a factor with this disease yet.  Still growers should follow good resistance management strategies including fungicide rotations and tank mixes to reduce the chances of any failures.  If you suspect resistance, contact one of us on the UF IFAS Blueberry Team to see about submitting a sample to test.  The disease is likely more difficult to control because some of our varieties are more susceptible to it than they are to rust or anthracnose by comparison.  For example, Arcadia is relatively resistant to rust and anthracnose, but does get some target spot.  I think in at least a few cases, poor results were partly due to application methods not getting good even coverage of foliage with fungicide product.  This is an area that needs some more research to test out air blast vs over-the-row and other types of sprayers.  In general, over-the-row sprayers utilizing air induction nozzles will get you better coverage than an airblast run down every other row.  It’s also worth revisiting the fact that fungicides don’t do us any good after symptoms show up.  They will help prevent new infections, but leaves already infected and showing symptoms will continue to fall after fungicide applications.  With target spot specifically, it is easy to overlook the small lesions within the vigorous regrowth.  By the time enough leaves have fallen under the bushes for most of us to take note, the disease has been cycling and building for some time.  Fungicide programs that start after significant disease has already occurred will require higher rates and more frequent applications to achieve the same control you will get from lower rates and longer intervals—but started before the disease becomes severe.

The UF IFAS Blueberry IPM guide lists products recommended for post-harvest foliage management of rust and anthracnose.  Target spot pathogen is sensitive to the active ingredients in Abound and Bravo, and in central Florida where anthracnose resistance is an issue; tank mixing captan with the Abound is still a good idea.  Rotating Bravo Weatherstik applications with Abound mixed with captan on a 14 day interval starting after harvest around the first of June and continuing through September would be a very solid plan of attack.  Where algal stem blotch is an issue, inserting monthly Kocide applications also would likely have some positive effect on target spot as well.  A DMI fungicide like Orbit should have some efficacy, and we need some more data on those.  Inspire Super has been used very successfully on tomato for target spot control, and also warrants some trial work for blueberry; however, that product shares one of its active ingredients with Switch and only 4 applications at the high rate of either product may be made before the seasonal limit of cyprodinil is reached.  Switch is probably more important as a bloom through harvest spray than Inspire Super will be for post-harvest target spot control given the other options listed here.  I don’t know if foliar applications of phosphorous acid products like Kphite or Agrifos will do much for target spot directly, but they do help with Septoria leaf spot and are good tools for preventing Phytophthora root rot during the summer months.

That can add up to a lot of applications.  Rather than calendar-based sprays, growers could scout for disease and either skip applications altogether when no symptoms are observed, or use lower label rates and longer intervals during good growing conditions to save product and money.  Keep in mind that we need to follow label instructions carefully.  Preventive applications before extended wet weather, like that associated with tropical systems, and just after hedging are good additions to a scout-and-spray program.  When scouting for foliar disease, look to the most susceptible varieties like Jewel to help predict when symptoms will follow and get severe on other less susceptible varieties.  Also, most farms have that one area too, maybe near the fence row in the lowest corner of the field, maybe where there’s a little more lime rock contamination… you get the picture… that’s where you want to pay close attention to what diseases are cooking.  Don’t ignore those harbingers.

In summary, we have one more potential disease issue to educate ourselves about and watch for after harvest. The list is getting longer, but we have some good options to consider.  I hope this information will help you keep your eyes on target spot when sighting-in a good post-harvest foliar disease management plan.

By PHILLIP F. HARMON, Department of Plant Pathology, University of Florida/IFAS

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