The Q&A Archives: fungus

Question: I have seven rose plants, all of them became infected with the leaves becoming spotted, turning yellow and dying, it is so early in the season, they were growing so well,all the leaves are almost gone,I just don't know what to do. And they are in the front of the yard,to the walkway.What can I do?

Answer: Although it might be a secondary infection, it sounds more like a typical black spot fungus that's affecting your roses. 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 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. Best wishes with your roses.

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