Spots on Leaves of Roses - Knowledgebase Question

Louisville, KY
Avatar for fstop
Question by fstop
May 16, 1998
I've noticed a few bright yellow spots on the leaves of a 'baby' rose bush I planted a few weeks ago. There has been little rain here, so I've been watering the bush almost every day. I did pinch off the affected leaves. (Ouch) Is there a less painful way that does not include 'chemicals?' Or is it not a problem?

Answer from NGA
May 16, 1998
There are a few reasons for these yellow spots. One could be spider mites. Check the undersides of the affected leaves for a "dirty" or "rusty" appearance. This is evidence of spider mites. Sometimes you will even see very, very fine webbing. If you see this tell-tale evidence, treat the rose with insecticidal soap. Another possibility is the beginnings of black spot fungus. Sometimes it starts out with yellowing leaves. Black spot thrives in humid, warm spots. To determine if you do in fact have black spot, check daily to see if more yellow leaves are appearing and if any have blackened areas. You can prevent black spot from returning by being very careful with watering practices, do not water in the evening, water early in the day so no moisture sits for very long on the foliage. Also, keep the area free from plant debris. Destroy any of those leaves you are plucking off. If you do have black spot on the roses (and it would be unusual this early in the year) you can treat with a baking soda solution. To make the solution, mix 1 tablespoon of baking soda and 2 1/2 tablespoons of ultra-fine horticultural oil spray with a gallon of water. The oil is slightly fungicidal and acts as a spreader-sticker, helping the baking soda coat the leaf and cling to the surface longer. The solution is a contact protectant so it stops fungi from attacking plant tissue, but doesn't cure the problem... Apply it as soon as the symptoms appear, repeating about every 2 weeks. Be sure to get under the leaves too. Another option is Rose Mosaic virus. There isn't much you can do about this one, it is usually spread when a plant is propagated from an infected plant. Most of the time it doesn't harm much, it's just unsightly. Pluck off and destroy affected leaves, this one can't spread to other plants so don't be concerned about that. Also note that reputable nurseries will replace diseased stock so keep that under consideration - you did mention they are new plantings. Finally, it may be as simple as overwatering. You mention you are watering every day, this may be too much. Always check the soil before you water and water only if it feels dry. A thorough watering once or twice a week is better than daily light watering.

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