Keep an eye on anthracnose infection risk model for blueberries

Annemiek Schilder, Michigan State University Extension, Department of Plant Pathology - June 21, 2011

Variable rainfall across Michigan results in varied anthracnose fruit rot risk ratings. Use the model on Enviro-weather to calculate your crop’s risk.

Due to considerable variability in rainfall amounts, different areas of Michigan may have experienced rather different anthracnose fruit rot risk ratings over the past few weeks. During dry periods, infection risk was often none or low. The only wetness that could have contributed to infection risk would have been dew formation at night or overhead irrigation. As we have reported before, the anthracnose fruit rot model is available on the Enviro-weather website and uses data from automated weather stations to calculate anthracnose fruit rot risk.

The model is applicable from bloom until harvest, assuming that inoculum of Colletotrichum acutatum, the fungus that causes anthracnose fruit rot, is present. If you have a susceptible cultivar and have had anthracnose fruit rot in the past, this is a reasonable assumption, particularly in the period from bloom until pea-size berry and during fruit ripening. The risk of infection is based on duration of fruit wetness (from dew or rain) and the average temperature during the wetting period, both of which are needed for spore germination and penetration of the fruit surface. The closer to the optimum temperature (77°F) and the longer the fruit stays wet from rain, dew or irrigation, the greater the risk of infection. The risk is indicated as “none,” “low,” “moderate,” or “high.” Anthracnose fruit rot symptoms are not visible until the fruit ripens.

You can look online to find out what the model indicates for the weather station closest to your farm. To access the model, go to:

Enviro-weather (www.enviroweather.msu.edu),
Pick the weather station of your choice by clicking on the dot on the Michigan map,
Go to “Fruit” (top green bar),
Under “Blueberry” on the left, select “Anthracnose fruit rot.”
You can change the weather station in the upper left corner and run the model for that weather station by pressing the “Execute” button.

The recommendation is to use “moderate” or “high” risk as a trigger for action if the bushes are not already protected by an effective fungicide. If green fruit was unprotected during an infection period, systemic fungicides such as Pristine, Abound, Cabrio or Switch can be applied within 24 to 48 hours (five days at the most) after the model has called an infection period to still get curative activity. In the case of ripe fruit, preventive application of fungicides is recommended in advance of major rain events since the fungus grows too rapidly in ripe fruit to have much post-infection efficacy.

Once a fungicide has been applied, the plants are protected for the next 10 to 14 days, depending on the length of the residual activity of the fungicide, and the model does not need to be consulted until the next spray may be due. If more than 1 to 2 inches of rain has fallen since the last application and infection risk is high, you may want to shorten the interval or reapply protectant fungicides. If there is no infection risk, such as during a dry period, the model can be used to stretch spray intervals. But keep in mind that if you apply overhead irrigation, this may trigger an infection period that will not be documented by Enviro-weather. In that case, try to keep the time that the plants are wet to less than six hours if possible.

Infection periods (since June 1, 2011)	Station	Start of Wetting Period	End of Wetting Period	Duration (Hrs.)	Avg. Temp (F)	Rainfall (in.)	Anthracnose risk	Wet hrs @ avg temp required for infection		Progress toward infection
West Olive
1	West Olive	6/15/2011 12:00 PM	6/16/2011 2:45 PM	26.8	57.9	0.83	Moderate	15.8	186%
Hudsonville
1	Hudsonville	6/10/2011 8:15 AM	6/11/2011 12:25 PM	28.3	58.2	0.17	Moderate	15.3	184%
2	Hudsonville	6/15/2011 12:25 PM	6/16/2011 2:55 PM	26.6	58	0.54	Moderate	15.3	189%
Grand Junction
1	Grand Junction	6/6/2011 8:25 PM	6/7/2011 8:25 AM	12.1	80.2	0	Low	9.5	132%
2	Grand Junction	6/9/2011 12:20 AM	6/11/2011 3:15 PM	63	60.5	1.26	High	14.3	511%
3	Grand Junction	6/15/2011 8:05 AM	6/16/2011 11:55 AM	27.9	60.9	0.16	Moderate	14.3	207%
4	Grand Junction	6/19/2011 9:30 PM	6/20/2011 3:00 PM	17.6	70.3	0.01	Moderate	9.9	182%
South Haven
1	South Haven	6/9/2011 9:25 PM	6/10/2011 2:50 PM	17.5	55.3	0.91	Low	16.9	117%
2	South Haven	6/15/2011 7:45 AM	6/16/2011 12:35 PM	28.9	53.4	0.5	Low	18.1	203%
3	South Haven	6/18/2011 4:35 PM	6/19/2011 7:20 AM	14.8	65.8	0.18	Low	12.1	138%
4	South Haven	6/19/2011 10:00 PM	6/20/2011 2:15 PM	16.3	63.4	0.02	Low	13	147%
Bainbridge/Watervliet
1	Bainbridge / Watervliet	6/9/2011 9:25 PM	6/10/2011 3:25 PM	18.1	58.6	0.64	Low	15.3	126%
2	Bainbridge / Watervliet	6/10/2011 11:50 PM	6/11/2011 3:10 PM	15.4	62.9	0.01	Low	13.4	120%
3	Bainbridge / Watervliet	6/15/2011 9:45 PM	6/16/2011 11:45 AM	14.1	61.1	0.06	Low	13.9	104%
4	Bainbridge / Watervliet	6/19/2011 9:45 PM	6/20/2011 2:55 PM	17.3	67.8	0.1	Moderate	11.3	159%
Lawrence/Teapot dome
1	Lawrence/Teapot Dome	6/6/2011 11:40 PM	6/7/2011 8:30 AM	8.9	77.6	0	Low	7.3	116%
2	Lawrence/Teapot Dome	6/9/2011 9:20 PM	6/10/2011 2:50 PM	17.6	53.7	0.63	Low	18.1	121%
3	Lawrence/Teapot Dome	6/15/2011 9:55 PM	6/16/2011 11:30 AM	13.7	53.5	0.01	Low	18.1	102%
4	Lawrence/Teapot Dome	6/18/2011 6:45 PM	6/19/2011 7:30 AM	12.8	67.1	1.16	Low	11.3	115%
5	Lawrence/Teapot Dome	6/19/2011 10:05 PM	6/20/2011 2:30 PM	16.5	66	0.07	Low	11.7	151%

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).