Common Rose Diseases

Advice on Common Rose Diseases

Fighting rose diseases effectively is a common concern for callers to Master Gardeners’ free gardening helpline, operated by the University of California Cooperative Extension.  In this third column, we consider some common rose diseases and offer suggestions.

Question 

What are the best ways to tackle powdery mildew, black spot, rust and other common rose diseases?

Answer  

Roses, for all their innate beauty, can be plagued with fungal diseases and pathogens.  Despite these challenges, many rose owners manage to grow lovely plants with little to no synthetic fungicides, especially in Ventura County’s dry interior valleys.   What’s the secret?

First off, be sure to select resistant rose varieties.  Then leave plenty of room between plants, as roses like plenty of air circulation.  Avoid overhead watering late in the day, so foliage can dry before evening.  It’s also a good idea to remove severely infested material promptly.

Here are some common rose diseases:

Powdery Mildew:   Do you have white or gray powdery growth on leaves, shoots, sepals, buds or petals?  It’s probably powdery mildew.  Sodium bicarbonate (baking soda) with horticultural oils might help.  Mix 4 teaspoons of baking soda per gallon of water with 1 oz of narrow range oil.  Apply during cool weather to avoid phytotoxicity.  Use this mixture sparingly, and avoid numerous applications with resulting runoff, as this can affect the soil pH and structure.  Also avoid getting the baking soda on blossoms, as it can distort them.  Removing infested parts also reduces the mildew.   Keep in mind that roses grown in sunny locations with good air circulation are less likely to have problems.  Landscape (scrub) rose varieties are among the most resistant.  Glossy-foliaged varieties of hybrid teas and grandifloras tend to be resistant as well. 

Downy Mildew:  Moist, humid conditions – especially in coastal areas – can cause downy mildew.   This fungus appears as interveinal, angular purple, red or brown spots on leaves, followed by leaf yellowing and abscission.  Reduce downy mildew by increasing air circulation around the plants, and avoid frequent overhead watering, especially late in the day.

Rust:  Small orange pustules on leaf undersides are a sign of rust.  This fungus likes cool, moist weather, and thrives during wet years.  Avoid overhead watering and prune back severely affected canes.  Be sure to clean up fallen leaves around plants, and be willing to tolerate minor damage.

Black Spot:  Another fan of wet conditions, this fungus can cause black spots with feathery or fibrous margins on upper leaf surfaces and stems.  Although black spot isn’t a common problem in California, miniature roses may be susceptible.   The baking soda solution mentioned for powdery mildew is effective, as is neem oil.

Botrytis Blight:  Spotted flower petals, large splotches on canes, and buds that don’t open are all signs of this fungus.  Botrytis blight is typically a problem during humid conditions and in coastal areas.   Modify irrigation and remove fallen leaves and petals.  Prune away infested canes, buds and flowers as well.

Stem Cankers:   Different fungi can cause ugly cankers on rose canes.  The cankers are brown, often with gray centers, or small, black, spore-producing structures on dead tissue.  To reduce cankers, avoid wounding canes and prune away diseased or dead tissue.  Always make pruning cuts at an angle in healthy tissue just above a node. 

Sunburn:  Are your rose canes blackened on the south and west sides?  They may have sunburn.  Reflected heat from rock mulch and vinyl siding can cause canes to sunburn.  Other causes are defoliation resulting from drought stress or spider mite pressure.

More Help:  Want more rose gardening tips?  The University of California Division of Agriculture and Natural Resources has free literature on common rose diseases and abiotic disorders (Pest Notes, Publication 7463).  Visit www.ipm.ucdavis.edu for a copy.

Free gardening advice is also available by emailing the Master Gardener Helpline at mgventura@ucdavis.edu