By Dr. Cassandra Swett
Anthracnose (also called ripe rot) in blueberry, has been quite severe this year, especially in highly susceptible varieties such as Bluecrop, Bluetta, and Blueray.
Anthracnose in blueberry is caused by Colletotrichum species, the very same troublesome fungi that also causes anthracnose fruit rot in strawberry and peach, bitter rot in apples, ripe rot in grapes.
Anthracnose is diagnosed by the sunken black lesions on the fruit, with little black dots
In blueberries, the fungus overwinters in twigs, buds and old fruit spurs that became infected last year. Spores infect flowers and ripe green fruit, so it’s important to protect from bloom through the green fruit stage. These infections don’t show symptoms until the fruit ripens or after harvest. The fungus also infects shoots and twigs and here you have the source of your inoculum for next year. These infections may cause lesions on the shoots, but infected wood and buds can also appear healthy.
If you put infected blueberries in a bag for a day, Anthracnose will make lots of orange gooey spore masses. If it has rained recently, the orange goo (the technical name) forms on the berries in the field.
In severely infected fields, you can prune out old and dead wood in the fall. Heavy pruning to open the canopy can both reduce humidity and improve spray coverage. Fungicides should be applied from pink bud to green fruit, and if you have high disease levels, continue to spray through harvest. The most effective fungicides include Switch, Abound, Pristine, Captan, Ziram, Omega 500, and Quash 50.
For an excellent integrated pest management guide for blueberries, see the 2013 Southeast Regional Blueberry Integrated Management Guide.
As we have discussed in previous articles this season, Anthracnose infection is weather dependent. Infection periods happen when we have at least 12 hours leaf wetness above 65 F. The weather stations in western Maryland and the eastern shore indicate that June had weekly infection events. So if you did not have full coverage through June, you may start seeing severe rot developing.
In Michigan, weather station data is used to evaluate risk of anthracnose for blueberries, to help growers decide whether they need to spray. We are testing to see whether diversified growers can use the same weather model to make decisions for Colletotrichum management in strawberries, blueberries, apples, peaches and grapes. This type of program not only lets you know when it’s important to protect, but can also help reduce the risk of fungicide resistance development and may be useful in reducing potential negative effects of fungicides on honeybee health.