May 1, 2024
Georgia strawberry growers battling fungus

Many strawberry sites in Georgia are now suffering substantial plant mortality as the season progresses.

Losses are not uniform, and some sites look pretty good. But unfortunately, many do not. If diseases continue to increase, this may be one of our worst production years ever. However, I hope that producers can still pull out some profits.

From the beginning, some growers received substandard plants, and some of these were already showing early signs of disease — especially Phytophthora root rot and the aggressive strain of Neopestalotiopsis. I attended a winter strawberry meeting in Arkansas, and much of the discussion centered around the poor quality of plants coming in from Canada; poor quality was equated to substantial rain during the time that tips were being harvested. In a court of law, this amounts to nothing more than hearsay evidence, but I did hear it from reputable sources, and it would make sense.

Phillip Brannen
Phil Brannen

This article originally appeared in the University of Georgia’s Strawberry Blog, written, by Phil Brannen, a professor in the Plant Pathology Department at the university. He has extensive experience with disease management programs in numerous cropping systems, and serves as the extension fruit pathologist for Georgia.

Phytophthora root rot has been very prevalent, and I think there are three reasons for this: early infections, possibly from transplants, significant rainfall events that have increased pathogen spread (Phytophthora spores swim), and likely mefenoxam (Ridomil) resistance in Phytophthora populations. When we have Phytophthora root rot, Ridomil is simply no longer working as well as it used to.

Though I would still use it at this point, I would incorporate more of the phosphonate fungicides like Prophyt and others as the season progresses — not trusting Ridomil alone. The phosphonates will also develop resistance as well, so use them according to label. Orondis cannot be utilized during harvest.

Neopestalotiopsis is also now very prevalent. Two or three years ago, I could tell you one or two nurseries to avoid when purchasing plants — those that had issues with Neopestalotiopsis in their production sites. Those recommendations have helped producers to avoid the disease for a few years. However, it is now difficult if not impossible to say which nurseries may be “clean” and which are not. While it is at least possible that we are getting infection from carryover inoculum in fields that were previously planted to strawberry, I think nursery plants are still a main source of initial inoculum in many cases.

Tips to battle Neopestalotiopsis

With wet, warm/hot weather, we are also observing more Neopestalotiopsis as the season progresses.

My recommendations for Neopestalotiopsis are as follows:

  • Avoidance through disease-free transplants — nursery source
  • Avoid planting varieties that are highly susceptible
  • Limit field operations, such as harvesting and spraying, when plants are wet
  • Employ hand, shoe and clothing sanitation between fields
  • Clean and disinfest equipment when moving between fields
  • Remove and destroy symptomatic plants to reduce inoculum and disease spread
  • Incorporate “efficacious” fungicides (Switch, Thiram, Rhyme, Tilt, Inspire) into the spray program
Neopestalotiopsis spots
Neopestalotiopsis damage on a strawberry leaf.

Jeff Cook, county agent and strawberry expert, has suggested that we might get better control with chemical fungicides if we use a flood nozzle that drenches the crowns, as this seems to be the area where the fungus resides. This sounds reasonable to me as well, so you might try that. It can’t hurt and it might help.

Removing individual leaves with symptoms is probably not practical and will not likely help that much, as the disease is likely already on the apparently healthy ones. Getting rid of infected fruit may likewise be difficult to achieve, but we still recommend it — for all diseases in general (Botrytis, anthracnose, etc.). Removing means removing from the field — not tossing in the row middle.

What about next year?

As much as possible, I would try to make sure that fields are thoroughly harrowed to break up the strawberry crowns that are left –— again as much as possible. Rotation would be ideal to let the survival structures break down and die, as it takes more than a year for this to occur. Removing old plants and destroying them would make sense, but it may not be cost effective.

We have tried solid-tarp fumigation with PicClor 60 and Vapam, followed by in-row fumigation later with another round of PicClor 60, and we still had carryover of the fungus. The cost of that degree of fumigation is just not sensible, so rotation is the best option. However, I do realize that many producers go back to the same location each year. If you can’t rotate, then I would advise use of Vapam and PicClor in your fumigation plans, as this would broaden the efficacy of fumigation against fungal pathogens. The absolute key to disease management will be acquiring disease-free and quality plants; this discussion has to start now. Bottom line, clean up the field, rotate if possible, and if not, fumigate and bring in good plants with no disease.

To add insult to injury, survey work for mefenoxam resistance and Neopestalotiopsis may have turned up another significant root rot pathogen, Phytopythium, but I will leave that one for another day. It was first reported in Florida in 2019 — possibly the first report in North America. Anyway, we will need to discuss that disease as we progress into the next season as well. I owe this recent detection to Alejandra Maria Jiminez-Madrid, the director of our Molecular Diagnostic Clinic in Tifton, Georgia. She is also intimately involved in strawberry disease research on Phytopthora resistance, Neopestalotiopsis detection and other fungal resistance surveys for strawberries.

For additional information on strawberry diseases and their management, please go to the IPM Guide at smallfruits.org or download the MyIPM app. Your local county agent is also your first point of contact for any questions or issues you might have. They can help you with sending samples for Phytophthora root rot confirmation and development of mefenoxam resistance, confirmation of the aggressive strain of Neopestalotiopsis, Botrytis fungicide resistance profiling with multiple fungicide classes, and resistance of the anthracnose fungus to QoI/strobilurin fungicides. Take advantage of these services; they are currently free until the money (grant funding) runs out.

 


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