By Ed Bradley
Good Rosarians try to keep the foliage on their rose bushes green, clean and healthy. However, in spite of our best efforts, sometimes we experience a “yellowing” of leaves. Unfortunately, those yellow leaves usually drop from the bush, sometimes leaving the bush with an inadequate amount of leaves to produce plant food (via photosynthesis), thus reducing the vigor and vitality of the bush.
Yellow leaves on roses are not normal (even in the autumn). Leaves are supposed to be green. Yellow leaves should be immediately examined in an effort to determine why they are yellowing, then take the appropriate measures to correct the situation. Let’s take a quick look at some of the more common reasons for yellowing leaves. This Part I will address damage caused by disease, insects/mites, too little or too much water, and physical damage. Part II will address yellow leaves caused by nutritional deficiencies.
Drought: In the heat of our summer, probably the most common cause of yellow leaves is simply not enough water. An indication that drought stress is occurring is that the leaves have a weathered bronze-green color. We tend to under estimate the amount of water a rose bush needs, and we tend to over estimate the amount of water we give it. A healthy rose bush requires one-to-two inches of water each week during the hot summer. Even in the winter, or especially during the winter, we tend to let roses really dry out, unless we are getting good and regular rains, because we sometimes think the roses don’t require watering because it is cool. Not so. The one-to-two inches of water should be delivered in applications of 1/3 to ½ inch; in other words, you will be watering about every-other day. The use of a good mulch will enhance the effectiveness of your watering program.
Fungus: The most common fungus here is blackspot; however, we can also be bothered by mildew. Almost everyone experiences blackspot in their garden, to some degree. Moisture on the leaves in early mornings will soften the leaf surface and activate blackspot spores. After the fungus spores penetrate the leaf surface, the black spots appear. The leaves will then turn yellow and drop off. An infected leaf is doomed. No matter what you spray with or how often you spray, the spotted leaf will not turn green again. There is a three-step solution: Remove as many of the infected leaves as possible, by hand. Spray weekly with a cleanup fungus-killer fungicide. Then spray with a preventive fungicide. (The prevention fungicide (Honor Guard, Banner Max, Rose Pride – choose only one) and the fungus-killer fungicide (Manzate, Mancozeb, Pentathlon – choose only one) can be mixed and sprayed at the same time. If this regimen is followed, the fungus problem should be eliminated in two to four weeks. Remember, however, that prevention is the key, and a regular spraying program must be followed to preclude recurrence.
Spider Mites: These little devils can defoliate a rose bush in a week or so. They climb onto the bush from the ground, or from an adjoining bush which is infected. The lower leaves should be removed from the bush to make it a little more difficult for the spider mites to get a foothold. A high-pressure water-wash is most effective in removing the spider mites. If severely infected, washing needs to be done every third day for nine days. (Why three times? You won’t get all of them with one washing. Eggs already laid will hatch, and each subsequent washing will blast away the new mites.) Effective control has also been achieved using a spray mix of liquid seaweed and agricultural molasses (1 Tbsp each per gal. water). There are also some effective miticides available; however, they must be applied to the underside of the leaves where the mites live and reproduce. (Avid, Floramite, and Shuttle).
Too Much Water: Strangely enough, too much water exhibits almost the same characteristics as too little water – wilting and yellowing of leaves. Wet, soggy soil deprives the plant of sufficient oxygen, and the plant is actually suffocating. Make sure your soil drains well. That’s why we promote raised beds and use sand in our rose soil mix. Sometimes a good soil will become compacted by heavy rains, or the mulch may prevent water penetration to the root zone. Aerate your soil frequently with a spade fork, rake, or similar tool without damaging the root system.
Damaged Leaves: Leaves that are broken, partially eaten by insects, or damaged by thorns on an adjacent cane will likely turn yellow and drop off. Some of this is normal, and there’s not much you can do about it. Obviously, you can kill the insects, and you can keep the extraneous growth cleaned out to minimize the damage from neighboring canes.
Dieback: Cane dieback is a mysterious disease which seems to defy explanation. There seems to be no common cause – fungus, bacteria, virus, whatever? The tip of the cane may simply begin to turn brown, and it will continue all the way to the origin of the cane (bud union). Other times, the entire cane will begin to turn yellow, followed by all leaves turning yellow, then the cane will turn brown (dead). When discovered, these should be immediately removed, all the way to the bud union.
Miscellany: The leaf on an older cane, directly below a newly-grown stem will eventually be aborted from the plant. This is normal and to be expected. All of the nutrients are going to the new growth, thereby depriving the older leaf of sufficient food. Additionally, the growth of the new stem literally pushes the older leaf away from the stem until it drops off.
Yellow leaves usually result from some kind of stress. Rosarians need to examine the circumstance to determine the cause, then take corrective actions to keep the bush green, clean and healthy. If you have an abundance of leaves dropping to the ground, keep these cleaned from under your bushes. They could very well harbor fungi or insects. Rake them away, or use your leaf blower to remove them. If you have difficulty determining the cause of yellow leaves on your bushes, call a Consulting Rosarian, or take samples to your local full-service nursery.
Finally, early autumn color on roses should be in the flowers – not the foliage.