Strawberries seldom encounter disease problems in Colorado. If your strawberry crop declines, you should first consider environmental or mechanical factors. Insects could be the culprit, but strawberries in Colorado are relatively free from insect pests as well.
If your crop is confronted with crown borers, leaf hoppers, aphids, earwigs, slugs or tarnished plant bug, fight back with a good insecticide. Malathion is effective against aphids, leaf hoppers, and quite a few other sucking and chewing insects. Use Sevin to control earwigs and beetles. Slugs may be controlled with a commercially prepared bait. Protect beneficial pollinating insects by not spraying during bloom.
If a disease is suspected, the cause is likely to be a fungus or virus. Bacterial diseases are uncommon in Colorado.
Diseases in a strawberry garden most often are controlled by removing an affected plant or plant part. If the disease is systemic — that is, it occurs throughout the entire plant — remove the plant. If a systemic disease occurs, it is likely to be caused by a virus or vascular fungal pathogen. In this case, you will want to avoid planting strawberries for three to five years. Such pathogens survive in the soil only for that length of time without a host.
If a fungus causes a leaf spot on your strawberry plants, and leaf removal has been ineffective or isn’t an option, spray the plants with a fungicide such as Captan. Always follow label instructions when applying pesticides.
For more information, see the following Colorado State University Extension fact sheet(s).