The Q&A Archives: leaves

Question: Why are the leaves on my rose bushes all of a sudden turning yellow?

Answer: Yellowing leaves can be a sign of water or heat stress, or it could indicate the first stages of a disease called black spot. Black spot is caused by Diplocarpon rosae, a fungus that overwinters on old diseased leaves and infected canes. Leaves are most susceptible while expanding. After fungal spores land on leaves, it takes at least 9 hours of leaf wetness for the spore to infect. Fungal fruiting bodies form in 11-30 days. A new crop of spores is produced and spreads to healthy portions of the plant by splashing rain or irrigation water. Spores do not survive in soil but do survive on all infected plant debris. The natural genetic variability of the fungus means roses found resistant in one location may be susceptible in another location due to the presence of different fungal strains. Also, resistant roses may become susceptible after a few years due to changes in the local fungal population. The symptoms of black spot begin with circular black spots, frequently with fringed margins. Yellowing and defoliation are common in susceptible cultivars. In wet weather, spots may become very severe and run together, making large irregular spots. Control practices include avoiding dense plantings (good air circulation all around the plant is important). Avoid overhead watering. Rake up and remove all leaves at the end of the season. You can try removing the yellowing leaves as soon as you notice them, but I suspect the canes might also be infected. A copper-based fungicide should stop the disease in its tracks. Or, since it's early in the season, you might try pruning your plants to remove any suspect wood and foliage. Best wishes with your roses.

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