Answer: This time of year I receive a great deal of mail from gardeners asking why their rose's leaves are turning yellow. With little or no more information than this color description, the possibilities of a correct diagnosis are slim. Leaf yellowing is symptomatic of a variety of problems, most of which can be pigeon-holed into four main categories -- enviro-mechanical problems, nutrient deficiencies, pest damage, and disease damage.
Yellowing leaves may also occur as a result of excessive irrigation, too much rain or improper drainage. Water displaces air in the soil. As a result, roots cannot support the rapid transpiration required during hot summers.
Conversely, if too little water is applied, a drought condition exists. This can cause equally devastating results.
Chief among reasons for yellowing leaves is "suffocation" caused by spraying. When chemicals and/or their surfactants clog leaf pores (the "stomata"), plant tissues cannot transpire moisture and essential gasses, they cease producing chlorophyll, cell walls deteriorate, and the leaves yellow and eventually fall from the plant. This symptom will be readily apparent, usually within a few days of the chemical's application.
Nutrient deficiencies such as nitrogen or iron can also lead to yellow leaves. If you haven't fed the roses, now is a good time to do so. Use a specially formulated rose food and apply in amounts as recommended on the label.
Best wishes with your roses!
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