Managing anthracnose in June-bearing strawberries
/by Sonny Murray
Anthracnose, which was once only a common disease in day-neutral strawberry varieties in August, is becoming an increasing nuisance in June-bearing varieties. Many of our popular June-bearing varieties (including Annapolis, Cavendish, and Kent) are quite susceptible to this disease. Disease development of anthracnose is favoured by moist, hot conditions.
Anthracnose in June-bearing strawberries overwinters on plant debris or in the soil. In early spring, anthracnose can develop on any plant part. Pink-to-orange-coloured lesions may develop on leaves, petioles, and runners in the early season as spores are splashed up onto developing plant parts. The spores produced in the centres of these lesions can then be moved to developing blossoms or berries by rain splash, machinery, or workers.
If blossoms are infected, often the flower cluster will die and dry up. If infections occur early in berry formation, the berry will fail to form and become hard and misshapen. The most recognizable symptom is the berry infection, which can occur at any stage during the berry formation and may only fully develop when the berry is in storage. Fruit infections will start as water-soaked spots on the fruit, quickly turning light brown. As the disease develops, the lesions will darken but remain tan in their interior. Observed under a magnifying glass, the spots within the lesions appear pink to orange. At this stage, under certain weather conditions (warm and wet with splashing), the disease can rapidly spread, making the whole crop unmarketable.
With climatic conditions working in favour of this disease, it can be very difficult to control. Fungicide application alone may not be enough.
In the following paragraphs, I’ll outline how to best manage anthracnose in our region.
USE CERTIFIED PLANTS
Not all nursery plant sources grow crops under strict phytosanitary programs. At the time of ordering, inquire whether the plants are certified and what the tolerances are for anthracnose and other viruses and diseases in the plant stock.
MONITOR
In a June-bearing crop, it’s worth scouting stands in the fall to assess the disease load that will be carried through on plant debris into the next spring. By doing this, growers have time during the winter to adjust the next year’s fungicide program if needed. In the spring, growers should begin anthracnose scouting as new foliage and berries develop. Early detection and fungicide application will go a long way to delay the spread of the disease.
MANAGE NITROGEN
High-nitrogen fertilization promotes the disease in several ways. Overgrown canopies are slow to dry out after irrigation and rainfall, giving the infection more opportunity to spread. High nitrogen levels in the plant can also lead to soft, susceptible fruit. Ammonium forms of nitrogen are readily accessible to the pathogen, so nitrate forms – such as calcium nitrate – may be safer alternatives.
REMOVE INFECTED FRUIT
It’s common practice to drop infected fruit between the strawberry rows. When dealing with anthracnose, it’s very important to remove infected fruit from the field so that it doesn’t serve as an inoculum source. This may not be practical for later picks on June-bearing strawberries, but in tunnel production or plasticulture (especially early season), it can really pay off.
ORDER OF OPERATIONS
Once the presence of anthracnose in a block is recognized, it’s very important that any field operations are performed in this block last. This can be said of many strawberry pests. Usually, younger plantings are cleaner. Start any field operations in the younger, cleaner blocks so that they may remain clean. This gives you an opportunity to clean and disinfect tools and equipment and change clothes before re-entering the new plantings. It’s also important not to work in fields where anthracnose is present while they’re wet, as this can aid in its spread.
IRRIGATION MANAGEMENT
Avoid excessive irrigation. Rain splashing from overhead irrigation quickly spreads the disease and provides the moisture needed for its infection.
ROTATION
Anthracnose, as with other diseases and viruses, can build up over the life of a stand. Taking the field out of production after two to three years of good harvest is just good practice.
CHEMICAL CONTROL
In 2021, Ontario’s agriculture department surveyed 10 farms across that province and confirmed the G143A mutation, which infers resistance in anthracnose to Group 11 fungicides. They include products such as Cabrio, Pristine, Merivon, Quadris Top, and Luna Sensation.
Resistance testing has not yet been performed in the Maritimes, so the status of resistance levels is unknown.
These products may be very effective at controlling other diseases, but if anthracnose is present, Captan/Maestro should be tank-mixed and used in rotation with Switch and Diplomat. Maintaining a good rotation of fungicides from different fungicide groups will delay resistance development on the farm. Plant pathologist Dr. Guido Schnabel, of Clemson University in South Carolina, often points out that once resistance develops, efficacy can’t be shifted back, as carrying the resistance gene doesn’t make the resistant population any less competitive.
ONGOING WORK
Perennia has made collections of anthracnose-infected strawberry plant material through the summer. That material will be used in testing through the winter to see which fungicides are still effective for control and will be recommended to growers next spring.
Perennia has also performed efficacy testing on two yet-to-be-registered fungicide products through Agriculture and Agri-Food Canada’s minor use registration program. Hopefully, these products will reach the market in the next couple of years, giving strawberry producers the tools they need to control this troublesome disease.
(Sonny Murray is a berry specialist with Nova Scotia’s Perennia Food and Agriculture Inc.)
