Mark Windham was a distinguished professor of ornamental pathology at the University of Tennessee, Knoxville. After retirement, Mark and his wife Karen love to walk beaches, visit with old rose friends, and enjoy grandchildren. These are a series of columns he is writing for various ARS rose publications. We will be adding about one a month for the next two years.
Mark’s Mayhems and Maladies
April 2025 Downy Mildew Can Ruin Your Day (and Garden)
We love Spring due the expectation of future blooms. Unfortunately, Spring is also the perfect time for downy mildew. M. J. Berkeley, the father of Plant Pathology, wrote on rose downy mildew control over 160 years ago. Though we have learned more about downy mildew since Berkeley’s time, it remains one of the more difficult rose diseases to manage. 
Downy mildew is caused by a fungal-like organism called Peronospora sparsa. Though it looks like a fungus, its genome is more similar to the human genome than to a fungal genome. Beside roses, it infects blackberries, dewberries, boysenberries, wild brambles (all Rubus species), and cherry laurels. Foliar lesions are red, maroon, purple or black. Leaf veins often border lesions and give the lesions an angular appearance.
Roses, infected the previous year, serve as reservoirs of the pathogen for early spring infections. The pathogen can sporulate and plants can be infected at 40F. However, symptoms will not appear till temperatures get into the 50s. Optimum symptom expression is in the low 60s. On most spring days, humidity levels are too low for downy mildew spores to germinate. However, after sunset, the air cools rapidly and relative humidity spikes, often to 90% or higher. This is the perfect environment for spore germination and for infection to occur, especially in crowded beds, unheated greenhouses, and high tunnels.
Pruning to allow good air movement around foliage is useful for downy mildew management. However, in cool, wet weather, this technique may have little effect. Rouging symptomatic foliage, picking up defoliated leaves, and inspecting new plants for downy mildew symptoms before planting are effective for reducing downy mildew impacts. Unfortunately, bio-soft products such as plant extracts and biological pesticides have not been effective for management. Agri-fos, Dithane, and liquid copper are labeled for homeowner use and are effective for preventing infections. Researchers in Germany have found resistance to downy mildew in wild Rosa species. However, resistance in cultivated roses is a long time away.
Feb 2025 – Phosphorus Is Like the Relative That Won’t Leave
Karen and I love company, but there have been times when dinner guests (including relatives) didn’t know when to leave. We wanted to go to bed! Dealing with high phosphorus levels in rose beds has left me with similar feelings. Phosphorus is necessary for good plant growth, robust flowering, plant disease defense mechanisms, and mycorrhizal growth. Unfortunately, phosphorus is often stationary in soils. At the end of one rose growing season, as much as 73% of added phosphorus can remain in the soil. After multiple years, phosphorus levels in rose beds may be very high. Clay or compacted soils can make these problems even worse. In acidic soils, iron and zinc can be bound to phosphorus making these elements less available to plants. Roses affected by phosphorus toxicity have reduced internodes, chlorotic (yellow) and smaller leaves, fewer and smaller blooms, and reduced root systems.
When working at the University of Tennessee, I was often contacted when rose beds had become ‘tired”. In “tired beds”, roses do not respond as intended to fertilization, watering, pruning, etc. “Tired beds” are often older rose beds in private landscapes and in public gardens. When I ask about soil test results, the standard answer was that soil tests had not been conducted. For many years, the roses had been fertilized with balanced fertilizers without matching fertilization rates to soil analysis recommendations. My diagnostic soil tests often revealed phosphorus levels were extremely high. My brother, who was an Extension ornamental plant pathologist at UT, once dealt with a soil sample analysis that revealed the grower would not need to add phosphorus for 2,000 years!
There are no silver bullets for correcting phosphorus imbalances. Some people try to avoid high phosphorus by putting in raised beds with “new” soil over their old, tired beds. If they do not change their fertilization habits, eventually, they will encounter the same mess again. One way to reduce phosphorus in beds is to utilize trap crops that deplete phosphorus. Lettuce, cereals, and legumes fit the bill. Beds that no longer support good rose growth can be used to first grow lettuce and then beans. Wheat can be used as a winter cover crop after the bean plants are removed. The key is removing the roots and stems of all trap crops before the plants die and nutrients are released back into the soil. Annual soil testing and reoccurring use of trap crops are used until phosphorus levels are low enough for good rose growth. Avoidance of manures and bone meal are recommended because both are high in phosphorus.
Soil testing. In Tennessee, it is easy to submit a soil sample to UT’s Soil, Plant, Pest Diagnostic Lab to get easy-to-decipher soil analyses for a modest price. In other states, the cost is higher and regrettably, some states’ Extension services no longer offer soil testing. If you are in a state that does not offer soil testing, check with your county agent to see what states accept soil samples from your area. They also may be able to recommend a private lab for this service.
Jan 2025 – Stem Canker Wants to Steal Your Roses!
There is nothing more discouraging than to sharpened your pruners and dressed for spring pruning only to find that brown/gray/black stem cankers have killed many of your roses’ canes. This lethal disease starts after roses have gone dormant. While you are snug under a quilt with hot chocolate or tea and assuming your roses have “settled down for a long winter’s nap”, your roses can be “stolen” by this disease. The suspects for causing this malady include the fungi Alternaria spp, Botryodiplodia theobromae, Botrytis cinerea, Coniothyrium fuckelii, Cryptosporella umbrina, and Trichothecium roseum.
Symptoms of stem canker usually start as small yellow areas on the canes. Cankers may have zonate (target like) patterns of gray and brown. Some cankers remain all year at less than an inch in length and are limited by nodes. Other cankers may grow across nodes and may be longer than 4 inches. Spores are produced on the surface of cankers and can be spread to other canes by wind or splashing rain. Canker fungi usually invade canes through freeze cracks, damage from canes rubbing together during wind events, pruning cuts, and other injuries such as humans pushing canes together while moving between roses.
Application of nitrogen fertilizers late in the growing season, leads to more and larger cankers. One or two weeks before your last Fall rose show or if not showing roses, then 4-6 weeks before the first frost day for your area, stop adding nitrogen fertilizer. Ensure you’re your pruners are sharp bypass pruners. Minimize late fall pruning. Anvil pruners may crush stem tissues at the cutting site and lead to a greater likelihood of canker formation. Fungicides have not been effective in preventing stem canker and are not recommended for canker management. During winter months, inspect your roses for stem canker at least twice and again at spring pruning. If cankers are observed, remove them immediately. When making the cut to remove a canker, cut at least one inch below the visible edge of the canker discoloration of pith tissue. Failure to detect and reduce canker incidence during winter months can lead to high lost of roses to this disease.
A. Stem cankers ravaging different dormant rose cultivars in early March in Knoxville, TN. Note the blacken stems with intermittent gray and brown cankers. B. A close-up of a terminal stem canker at an old pruning cut. The small black flecks on the brown tissue (look like black pepper flakes) are fungal fruiting bodies that release spores.
