Anthracnose fruit rot prediction model to assess risk of infection in blueberries

Use MSU Enviro-weather’s anthracnose fruit rot prediction model to guide fungicide spray timing in blueberries.

Anthracnose fruit rot risk based on fruit wetness and temperature. Low: 1-10% infection incidence. Moderate: 11-40% infection incidence. High: 41-100% infection incidence.

Anthracnose fruit rot risk based on fruit wetness and temperature. Low: 1-10% infection incidence. Moderate: 11-40% infection incidence. High: 41-100% infection incidence.

Anthracnose fruit rot, caused by the fungus Colletotrichum acutatum, is the main fruit rot disease in blueberries. Between fruit set and harvest, Michigan State University Enviro-weather’s anthracnose fruit rot prediction model can be used to get a sense of disease risk. This model is based on leaf wetness and temperature, which are critical factors in infection. For anthracnose fruit rot, the optimum temperature for development is 77 degrees Fahrenheit at which a minimum of six to eight hours of fruit wetting is needed for infection to occur (see Figure 1). However, with increasing wetness duration, the number of spores that germinate – and therefore the infection risk – increases. The model is most useful during dry seasons as infection periods tend to be relatively rare and spray intervals can be extended accordingly, resulting in fewer fungicide applications.

The disease prediction model for anthracnose fruit rot infection is available on the Enviro-weather website. Remember that wetting of plant tissues can be caused by rain, dew or overhead irrigation. Thus, if you use overhead irrigation, you need to take that into account because the model does not cover this. The model uses weather data collected by automated weather stations that are present in Michigan’s main blueberry-growing areas, including Grand Junction, South Haven, Fennville, West Olive, Watervliet, Hartford and Benton Harbor. Environmental conditions are recorded every 5 minutes and updated on the website every 30 minutes.

Recorded weather data can be freely accessed on the Enviro-weather website by clicking on the appropriate weather station location. Choose the closest weather station to your farm and look under “Fruit,” then “Blueberry” and then “Anthracnose fruit rot.” If your farm is located between weather station locations, MSU Extension advises to look at the conditions at both weather stations. Always remember that weather conditions are variable, so local observations on your own farm are the most accurate, particularly where rainfall and leaf wetness are concerned.

How to use the anthracnose prediction model

The model can be used from bloom until harvest and assumes inoculum is always available. This assumption may not be true, but represents the “worst-case scenario.” From spore trapping experiments, we know that in rainy years, spore counts peak around bloom and early fruit development and again at fruit ripening. However, during warm, dry summers, the pathogen may be active at low levels throughout the summer. Since the model uses recent (=past) weather data, it will indicate if an infection period has occurred. An infection period is recorded when the minimum wetness and temperature conditions for infection have been met, which is reached at 100 percent progress towards infection. This will represent a “low” risk of infection. The percentage value continues to increase with prolonged wetness duration, eventually reaching “moderate” risk and “high risk.”

To keep tabs on the situation, it is helpful to check the model frequently. The model is also educational in that users learn to recognize what weather conditions constitute a moderate and high risk of infection. Since we don’t yet have the ability to use forecasted weather data to predict future infection periods, growers can use the model to learn to estimate risk based on the local weather forecast.

Anthracnose prediction model

Enviro-weather’s blueberry anthracnose risk for South Haven, Michigan, as of June 21, 2016. View larger picture of model.

If an infection period has occurred and the plants were not already protected by a fungicide, a systemic fungicide should be applied within 24 hours to stop the infection in its tracks, particularly if the indicated risk is moderate or high. Locally systemic fungicide choices are Abound or Pristine, which are effective against anthracnose fruit rot unless fungicide-resistant strains are present. It may be advisable to tank-mix with Captan if disease risk is high. Aliette, Phostrol and Quash are more systemic, which may give them an advantage in terms of curative activity even though they are inherently somewhat less effective than strobilurins.

On ripe fruit, the infection progresses very rapidly and is already well-established after 24 hours, so if a curative spray cannot be applied immediately following the infection period, protective sprays should be used in advance of the next expected wetting period to protect immature fruit in the clusters. Once a fungicide has been applied, you can consider the plants covered for at least 10 days unless frequent rainfall occurs, which may require reapplication sooner.

You can also apply a protectant or systemic fungicide (e.g., Captan, Ziram, Omega, Switch, Pristine or Abound) before a predicted rain event based on the local weather forecast and your own estimate of projected wetting duration. This is a good idea if several days of rain or windy conditions are predicted, such that it may be difficult to get into the field to apply a spray within the limited timeframe after an infection period has occurred. In addition, it is important to alternate different chemistries because using strobilurins repeatedly can increase the risk of fungicide resistance development. Pay attention to the pre-harvest intervals for the respective products.

For organic growers, Sil-Matrix (potassium silicate) is the best option for protection of blueberry fruit against anthracnose. This is a contact material, however, and will need to be applied in advance of infection. If neem products are used for insect control, these will also help against anthracnose fruit rot.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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