Rose Diseases: Fungal

Rose Diseases: Fungal

Mark Whitelaw


Fungal rose diseases are experienced by most rosarians and can occur in most any garden. Apart from the rare instance, all fungal diseases are culturally induced. That is, the fungal spores find a home in the garden by locating environmental conditions favorable to their reproduction and infestation.

The most common fungal rose diseases are:

Blackspot (c. M. Whitelaw)Blackspot Sometimes called "Leafspot," this disease is the most common to all rosarians worldwide. Serious infestations can greatly weaken plants, producing stem die back and unproductive growth. A weakened plant will most likely experience winter damage in colder climates.

Damage usually begins as a black or brown splotch. Shortly after, the leaf tissues surrounding this spot turn yellow. This is because fungi feed only on dead tissue. In this case, the fungus exudes a chemical called ethylene to kill the leaf tissue as it advances; the result of this is the destruction of chlorophyll in the cell living tissue and the creation of a golden "halo" or yellow surface area indicating the dead tissue.

The spore germinates in as little as nine hours on moist leaf tissues where temperatures remain between 70-80° F (21-27° C). Under most conditions, the leaves must remain wet a minimum of seven hours before infection can occur. Within 15 to 37 days, visible colonies can be observed from a single spore's infection.

The pathogen is susceptible to fungicides within the first 90 hours but may not be noticed. For this reason, preventive fungicide applications are best made during susceptible periods of the year if these are your preferred control means. For other controls, see Rose Diseases and Their Control.

Some roses are more susceptible to Blackspot than others. According to the American Rose Society, the highly susceptible genes of Rosa foetida were transmitted through R. foetida persiana and "Harison's Yellow" to the Pernetianas, the parents of our yellow roses. For this reason, most yellow roses are highly susceptible to Blackspot.

Powdery Mildew Arguably the most pervasive and destructive of all the fungal diseases, Powdery Mildew will attack all rose species in a wide variety of climates. Severe, untreated infestations will cause distorted new growth, often killing growth tips and buds.

Powdery Mildew's advanced stages are easily identified by white or gray "flocking" on leaves, stems, buds and thorns. Early symptoms include slightly raised blisters on the upper leaf surfaces.

Spores optimally germinate in as little as three hours when temperatures are 71° F (22° C), but can reproduce when they are as low as 42° F (6° C). The spores are most viable when relative humidity is about 80 percent. When temperatures exceed 70° F (21° C) for more than 48 hours or 90° F (32° C) for more than 24 hours, the spores are killed. Although high humidity increases spore production, water films actually suppress their growth. In short, Powdery Mildew finds a comfortable home where roses are subjected to warm, cloudy days and cool, humid nights.

Proper air circulation is a must for control. Where poor circulation exists, infestations can be severe. If fungicides are selected, they should be applied on a regular basis with particular attention paid to periods of rapid growth. For other means of control, see Rose Diseases and Their Control.

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Downy Mildew This disease is most commonly confused with Blackspot. But unlike its "cousin," the damage is first noticed at the top, new-growth areas of the plant instead of the lower leaves as with Blackspot. Infected leaves develop purple-red, irregular splotches. Mature stages of the disease manifest as gray fuzz or "down" on the undersides of leaflets.

Downy Mildew is happiest under cool, moist conditions where air currents and splashing water can transmit the spores.

Collection and destruction of diseased leaves is paramount. For other controls, see Blackspot above.

Rust This disease is the nemesis of rosarians primarily along coastal regions. There are nine species of this disease, however, and they can show up most anywhere given the right growing conditions. Rust is characterized by bright orange, powdery pustules, usually on the underside of leaves. Eventually populations increase, the pustules enlarge, turn brown and become visible on both sides of the leaves. In severe infestations, stems are attacked and new growth becomes distorted.

Optimum conditions are continuous moisture for two hours at 65 F (18 C). The rainy seasons of spring and fall favor development. Spores are carried about by wind and moisture and will overwinter in an infested garden.

For control, see Blackspot above.

Canker Occurring in the garden more frequently than some rosarians suspect, this disease gains entry through tissue wounds caused by crossed canes which have rubbed together, unsanitary pruning practices, or even from cutting blooms. Although usually confined to stems and canes, Canker can attack any portion of the rose, even crowns.

The cankers begin as yellow or red spots on the bark which eventually turn brown as the disease progresses. The bark tissue within the canker will eventually dry and shrivel. If infestation is severe, the canker will surround the cane or stem and kill that portion above the infected area. For northern gardeners who cover their roses during winter, these cankers may first appear as blackened areas. After a week or so of exposure, however, they return to their characteristic appearance.

Correct cultural practices and sanitary pruning procedures will reduce the risk of Canker. Do not permit stems or canes to rub against each other. Remove one or the other during annual pruning. When pruning canes, prune close to the node ("budeye"). Failing to do this permits the cane to die back, rot and create a condition favorable to Canker spores. If infestations are severe, remove the offending shrub.

Botrytis Many rosarians don't even recognize this fungal disease. It attacks blooms and canes of established plants, and the bare roots of improperly stored or shipped plants. It appears as a gray-brown fuzzy mold. Buds may fail to open or open only partially. A gray-black lesion usually appears on one side, extending from the bottom of the bud down the bloom stem. On partially opened blooms, petal edges may appear brown and soft, an appearance similar to larval thrips damage or "balling."

Botrytis fungal spores favor continued wet weather and darkened conditions. Watch mail-ordered, bare-root roses carefully. If fungicides are selected, those commonly used for Blackspot will suffice for control. However, smart cultural practices will do much towards control. Plant roses where they will receive early morning sun, thus drying blooms and buds of dew. When symptoms first appear, remove blooms and bloom stems as soon as possible to reduce spore proliferation.

Anthracnose This fungal disease overwinters in the lesions of older rose canes. As wet conditions occur in spring and early fall, spores attack nearby canes and leaves. Symptoms appear as round, quarter-inch (six-mm), dark black, brown or purple spots on leaves. As these spots mature, they turn tan or light gray in color with dark red around the margin. The leaf tissue within these rings may separate from the underside of the leaf, leaving a tissue paper effect. Symptoms on stems and canes are raised brown or purple spots.

Control is best achieved by removing the entire shrub if infestation is severe. Otherwise, infected canes should be pruned away in early spring before leaves appear.

Next time, we'll answer the question, "When is a rose disease not a disease?"

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