Utah Pest News Spring 2011

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Powdery Mildew in Your Backyard – Part II


  Severe infections of powdery mildew can result in a plant of poor vigor and cause losses in fruit yield, as shown on watermelon (top).  It is best to monitor plants and look for young powdery mildew lesions forming on leaves to determine when to treat, such as on these cucumber leaves (bottom).


There are two powdery mildews that can occur on melons, squash, cucumber, pumpkins, and watermelon.  Without a microscope, both look alike.  Podosphaera xanthii (syn. Sphaerotheca fuliginea) is a mildew that has developed several races that infect some cucurbit varieties but not others.  Seed companies may mention whether a variety is resistant to powdery mildew and to which race.  For example:  bean variety ‘A’ is resistant to race 1 but not race 2.  If race 1 is found on a neighboring bean variety ‘B’, bean variety ‘A’ would not get powdery mildew.  However, if race 2 of this mildew is present, then bean variety ‘A’ would be infected.  Podosphaera powdery mildew occurs during warmer months.

The second powdery mildew is Erysiphe cichoracearum.  It does not have races and infects all cucurbits except watermelon (which is susceptible instead to Podosphaera xanthii).  E. cichoracearum also infects many weeds and some ornamentals that could provide inoculum for cucurbits.  E. cichoracearum is more frequently observed during spring and early summer when temperatures are cooler. 

Severe powdery mildew infection can result in yield loss.  Both powdery mildews cause damage to the plants by reducing photosynthesis.  Once the leaves are covered with white mycelium, they absorb less sunlight and are not able to produce enough sugars to sustain plant and fruit growth.  In addition, heavily infected leaves become necrotic (turn brown and die) and fall off, which can result in sunburn of fruit.

The best management is the use of resistant varieties.  Sulfur products work very well when they are applied just as the first spots of mycelium (threads of fungal growth) appear on the leaves.  The treatment has to be repeated throughout the growing season as new spots appear.  Sulfur should not be applied at temperatures above 90°F, as the leaves will burn.  It is best to apply sulfur either in the evening after temperatures cool down or very early in the morning.  Potassium bicarbonate is also very effective but needs to be applied often and does not have residual activity.  Removing infected plant material with mycelium and spores results in fewer spores present to cause new infections.


Powdery mildew on ornamentals affects mostly the aesthetic value of the plants.  Ornamentals such as lilac, maple, rose, columbine, dahlia, delphinium, and phlox are frequently infected with powdery mildew in late summer and early fall.  There are many different powdery mildew species that infect ornamentals.  Each of them will infect one or more plant species.  For example, Sphaerotheca pannosa infecting roses can also infect photinia and Erysiphe cichoracearum infects many ornamentals in the composite family including zinnia, phlox, chrysanthemums and dahlias.  Treatment options are the same as with powdery mildew of cucurbits.

-Claudia Nischwitz, Plant Pathologist

For further information, visit these online resources:

Part I of this Topic

Cucurbit powdery mildew, Cornell University

Cucurbit powdery mildew, UC Davis

Ornamental powdery mildews, Utah State University 

Ornamental powdery mildews, UC Davis