Consulting Rosarians
History of MRS
MRS Public Garden
Bronze Medal Awards
Standing Rules
Reimbursement Form

Join Marin Rose Society!
Newsletter sample
Join the ARS

Upcoming Events:

Rose Culture:
Garden Good/Bad Guys
Great Roses
Annual Rose Care
Monthly Rose Care
Rose of the Month

Rose Purchasing:
Buying Roses
Mail Order Sources

Rose Shows:
Annual Rose Shows
MRS Trophies

Poet's Corner
Rose Books
Rosey Links
Site Map
Members Only

Chart of Pests on Flowers

by Nanette Londeree, Master Rosarian

For us rose lovers, it’s time for the big Spring Bloom – the floral equivalent of the World Series and Super Bowl rolled into one! All our hard work over the past months pays off big time with bountiful, gorgeous blooms; gleaming hybrid teas, floribundas and shrubs cloaked in colorful, perfumed blossoms, magnificent climbers tumbling over walls, trellises and arbors and sweet miniatures sporting vivid bursts of color. Finding leaves dotted with rust or blackspot pelting the ground around the blooming plant, new buds a frosty white from powdery mildew, or shot-holed leaves infected with anthracnose, can put a real damper on the glorious season. Rose blooms don’t look quite the same framed with damaged or unsightly foliage. Those telltale symptoms are the results of infectious disease, the major plague of the rose gardener. So now what’s bugging your flowers (or leaves, canes and roots)?

Plant diseases are caused by living organisms called pathogens. For infectious disease to occur, three critical factors or conditions must be present – a susceptible host plant, a pathogen and the right environmental conditions. The relationship of these three elements is called the disease triangle. If any one of these elements is missing, disease will not occur. Understanding the disease cycle can help you find ways to reduce or eliminate infections.

Disease Triangle

Susceptible Host Plant - A plants ability to be infected is dependent on both genetics and its overall health and vigor. You’ve probably heard about roses that are considered “disease magnets,” varieties that are more likely to get a dose of powdery mildew or rust than others (think ‘Sterling Silver’). That proclivity to disease is in their genes. Selecting rose cultivars that are disease resistant is a major step in disease prevention. Keeping plants healthy through good cultural techniques increases their ability to fend off disease.

Pathogens - Plants, like humans and animals, can be infected by a pathogen - a bacteria, fungus, virus, mycoplasma or nematode that results in disease. These infectious agents are specific to a particular plant species, genus or family. The powdery mildew dusting your roses is caused by a different species of fungus (Sphaerotheca pannosa) from the one that produces similar symptoms on dahlias (Golovinomyces cichoracearum) or zucchini (Sphaerotheca fuliginea).

Pathogens are generally carried to the host plant by water (rain and irrigation), wind, insects, birds and people. Splashing water carries spores from infected to uninfected leaves; wind blows them from plant to plant. Insects and birds can inoculate a plant when feeding, and the errant gardener can transfer the infectious agents via plant handling and tools. The easiest way to reduce the opportunity for the pathogen to move in your garden is to maintain good garden sanitation - clean up and remove infected debris, and sanitize tools often when working with infected plants.

Environmental Conditions - Specific environmental conditions must exist for pathogens to cause infection and these vary for different pathogens. Moisture levels and temperatures are generally the most critical. Blackspot and rust both require available water on the leaves for defined period of time to infect the plant, while those same conditions actually prevent powdery mildew. Downy mildew thrives in high moisture and mild temperatures – in our climate, it usually disappears in summer when temperatures hit 80°F or higher and the humidity is low.

While you can’t control the whims of Mother Nature, planting your roses in sunny sites with lots of air circulation and good drainage, and irrigating in a way that minimizes moisture on the leaves provides environmental conditions that may not favor the development of infection.

Which of the three elements in the disease triangle can you alter to reduce or eliminate disease that may be lurking in your garden?

Photos of bullnosing courtesy of Wisconsin State University website
Photo of thrips damage by Nanette Londeree
All other photos courtesy of Baldo Villegas’ website at

Return to the Main Page

Search WWW Search

Contact us
© Marin Rose Society
All Rights Reserved
Last Modified: 08/06/2013