Use Anthracnose Fruit Rot Prediction Model To Assess Risk Of Infection

 

 

Most fungi need free moisture and moderate temperatures to infect plants. Knowing the specific environmental requirements for spore germination and infection by a pathogen can be helpful in predicting disease development. In the case of anthracnose fruit rot, the optimum temperature for the pathogen (Colletotrichum acutatum) is about 77 degrees Fahrenheit at which a minimum of six to eight hours of fruit wetness is needed for infection to occur. However, with increasing wetness duration, the number of spores that germinate – and therefore the infection risk – increases. You can see from the graph that at higher and lower temperatures than the optimum, the fungus does not grow as well and therefore needs the tissue to stay wet longer in order to accomplish its feat.

 

 

 

 

Low: 1-10% infection incidence

Moderate: 11-40% infection incidence

High: 41-100% infection incidence

 

A disease prediction model for anthracnose fruit rot infection is available on the Michigan State University Enviro-weather. The model is based on leaf wetness and temperature requirements for fruit infection. Wetting of plant tissues can be caused by rain, dew or overhead irrigation; if you use overhead irrigation, you need to take that into account because the model does not cover this –that may not be as much of an issue this year. 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 five minutes and are updated on the website every 30 minutes.

 

Recorded weather data can be freely accessed online by going to the MSU Enviro-weather website and clicking on the closest weather station to your farm. If your farm is located between weather station locations, it is advisable to look at the weather data recorded by 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.

 

 

The anthracnose fruit rot model can be used from bloom until harvest and assumes inoculum is always available. This may not be true, but represents the “worst-case scenario.” From spore trapping experiments, we know that in a rainy year, spore counts peak around bloom and early fruit development and again at fruit ripening. However, during warm, humid conditions, 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.

 

If an infection period has occurred and the plants were not protected by a fungicide, a systemic fungicide should be applied within 12-24 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, while Aliette, Phostrol and Quash may be somewhat less effective than the strobilurins but more systemic, which may give them an advantage in terms of curative activity. On ripe fruit, the infection progressed more rapidly and is already well-established after 24 hours, so if a curative spray cannot be applied immediately following the infection period, protectant sprays should be used in advance of an expected wetting period to protect immature fruit in the cluster. Once a fungicide has been applied, you can consider the plants covered for 10-14 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, Abound) before a predicted rain event based on the local weather forecast and your 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 time frame 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), Regalia (giant knotweed extract) and neem oil are the best options for protection of fruit against anthracnose.