Black spot is one of the most common and destructive rose diseases that we see in Texas. Some roses are resistant to the disease but all species and cultivar are affected. Defoliation, decreased quality and quantity of blooms, and increased susceptibility to other stresses reduce overall plant health and cause significant losses to roses.
- Small brown to black leaf spots outlined with feathery, irregular borders appear.
- Spots may occur on either side of the leaves and on the canes, but usually show up on the upper leaf surfaces.
- As the disease progresses, chlorotic (yellow) rings develop around each spot,growing and merging and eventually causing the entire leaf to yellow before dropping prematurely (Fig. 1).
- Symptoms first appear on the lower plant parts, advancing upward until the roses become completely defoliated.
- The fungus does not kill infected roses outright, but the repeated loss of leaves weakens them so they produce fewer blooms and are generally less healthy.
- The fungus also makes the plant vulnerable to other stresses such as secondary infection by other diseases, adverse or extreme environmental conditions, and winter injury.
- Clean up dead leaves and prune infected canes to help reduce inoculum for the next season.
- Plant varieties that are resistant to the disease.
- Use watering practices that lessen leaf wetness since D. rosae spores need moisture to germinate and infect the plant. Promote rapid drying of the leaves by avoiding overhead watering and watering in early to mid-morning.
- Encourage good air circulation and prevent the spread of the disease to adjacent plants by not planting too densely and by planting roses in full sun.
- Prune and dispose of damaged and infected plant parts and remove infected canes and leaf litter to prevent re-infestation the following spring.
- Fungicides are available to help manage black spot. Conventional (products containing
- chlorothalonil, propacanazole or myclobutanil) or natural (products containing neem) fungicides are effective when used appropriately. Refer to product labels for proper rates and use.