By Baxter Williams
"Even in the dead of winter, I have color in my rose garden—brown canes, black spots and yellow leaves." Perhaps that description has also fit your garden from time to time, as it has mine. But it might be that this "color" is resident for most of the growing season. What is the reason, or reasons, for this discoloration?
The answer is, of course, that there are several reasons for yellow leaves. When you stop and think about them, the causes can be divided into two groups: problems in the root zone, and problems with the leaves themselves. Perhaps our analyses should start with the roots.
In the root zone, problems can usually be traced to dryness, too much water, or pH imbalance. When the soil is allowed to dry out, it becomes drier than your rose’s roots, which cannot then take up nutrients or life-giving moisture. If the soil is dry, moisture will leave the roots and move into the soil, desiccating the plant and shutting down its circulatory system. The leaves will do their best to retain cell moisture, but the plant will abort them to prevent further loss through the stomata (pores). The leaves are likely to change to a bright yellow, beginning low on the plant and uniformly distributed. The problem solution is water, and lots of it.
Curiously, too much water in the root zone excludes needed oxygen and promotes rotting of the roots. The old adage, "Roses don’t like ‘wet feet’," is true. The solution is two-fold: raised beds (12 inches above the existing soil) or beds with proper drainage, and replacing packed soil with some that is more porous.
Rotting is most likely caused by phytoptera, which is a soil fungus. Phytoptera can be eliminated by applying a soil drench of Subdue MAXX or Aliette, but that might not save an already-infected plant. Phytoptera can also attack an otherwise seemingly healthy plant, causing its decline (or demise) while not affecting adjacent plants.
A large excursion of pH in the root zone, say below values of 5.5 (much too acid) or above 7.0 (too basic, or "alkaline"), prevents the uptake of nutrients by "locking up" the atoms. Nutrient atoms must be ionized (electrically charged by removing a particle) to be ingested, and high or low pH cancels the ionic charge. Visible effects are slow to manifest themselves, and are likely to show up as loss of chlorophyll (less dark green) in the leaves, and eventual yellowing. The solution for too high pH is to add a soil acidifier in small amounts, checking between each adjustment, until pH is between 6.5 and 6.8. For too low pH, add dolomite lime, checking between adjustments until pH is between 6.5 and 6.8. Note: Raising the pH takes chemical reaction time (days or weeks) because of slow chemical reaction time in the ground.
With respect to leaf yellowing caused by maladies in upper (above the roots) plant parts, be aware that some normal leaf drop will occur. The leaf at the node from which a lateral cane grew will drop eventually, when its effectiveness diminishes. It could very well be the only yellow leaf on the bush. It will happen; no solution.
A leading cause of leaf drop is blackspot fungus. Almost every variety of roses is susceptible to this disease. Blackspot is insidious in that every black spotted leaf is sure to drop: they are fatally damaged. And blackspots are deceptive: Their presence indicates damage that was done to the plant 10-15 days prior to their appearance. The corollary to this idea is profound: if some of the leaves on a plant show blackspot, then other non-spotted leaves are probably already damaged, that damage being not yet visible. The solution is to spray the bushes with a contact fungicide mixed with a penetrant type fungicide, and to continue to do so every week. After 4-5 sprayings, the contact fungicide can be discontinued.
Powdery Mildew and Rust funguses can also defoliate a plant over a relatively short period of time. Powdery Mildew causes whitish spore masses, both on leaf surfaces and on other tender plant parts, which precede yellowing and leaf drop, usually by several days. Fortunately for us, rust is not native to our area, and is only contracted from infected plants shipped into the area. Solutions are the same as for treatment of blackspot.
Manual damage from adjacent thorns or other abrasive sources is possible. If severe enough, the plant will abort the leaf. Solution: remove the source.
Spider mites can, and will, cause yellowing and leaf drop. Damage from spider mites is nearly always first evidenced by a reduction of chlorophyll (a gray-green cast) in the centers of the leaflets. Eventually the leaflets will yellow and drop away. Solution: Destroy the spider mites colony underneath the leaflets by using high-pressure water washes at 2-day intervals for a week, or by spraying with Avid.
Water’em, feed’em, spray’em, eliminate harmful insects, and keep the soil pH correct, and you’ll be enjoying the Queen of Flowers for most of the year.
Editor's Note: This article reprinted from the May 2003 issue of "The Rose-ette", newsletter of the Houston Rose Society, Patsy Williams, editor.