Win the Battle Against Powdery Mildew

Powdery mildew is one of the most easily recognizable diseases of landscape and garden plants.  While most fungal pathogens are favored by wet weather, powdery mildew pathogens favor high humidity but not wet conditions.  Shady areas, areas with poor air circulation, and low areas that trap damp air are ideal environments for disease development.

Powdery fungal growth is an easily recognizable symptom of powdery mildew disease.

A wide range of plants are susceptible to powdery mildew:  annuals, perennials, shrubs, ornamental and fruit trees, small fruit, and vegetables.  While symptoms appear similar on most plants, powdery mildew fungi are usually host-specific.  For example, powdery mildew of dogwood cannot infect pumpkins or cucurbits, and powdery mildew of cantaloupe will not infect phlox or rose. 

Typical symptoms of powdery mildew include dusty fungal growth on surfaces of leaves and on young succulent plant tissue (Photo 1 & 2).  Powdery mildew may occur as isolated blotches or cover entire leaves, stems, buds, or flowers.  Early infections can lead to leaf stunting, curling, or other deformation (Photo 2).  Infected fruit may become disfigured or fall prematurely (Photo 3).  In the fall, small fruiting structures that resemble pepper flakes may be visible to the naked eye (Photo 4 & 5).  These structures contain overwintering spores that serve as inoculum for the following year.

Infection of young growth may result in stunted or deformed tissue.

Management of powdery mildew begins with prevention.  Plants should be properly spaced and thinned for improved air circulation and more rapid leaf drying.  Resistant cultivars are available for crabapple, dogwood, phlox, zinnia, cucurbits, and several other plants and are an excellent means of disease prevention.

Fruit infections ultimately lead to yield loss.

Fungicides usually are not warranted when cultural practices are implemented.  Early season infection, however, may require one or more applications of effective fungicides.  Homeowners have a wide range of fungicides available.  However, they should carefully read labels to confirm that the selected fungicide may be used legally on select plants or that the fungicide is suitable for edible plants.

Fungicides effective against powdery mildew include:

·         Chlorothalonil – broad spectrum protectant; ornamentals, some fruit and many vegetables

o   Bonide Fungonil concentrate or Fungonil RTU

o   Ortho MAX Garden Disease Control

o   Sevin Daconil

·         Myclobuanil – systemic; ornamentals, most fruit and vegetables

o   Spectracide Immunox (only Immunox Multi-purpose Fungicide Concentrate is labeled for use on fruit and vegetables)

·         Propiconazole – systemic; ornamentals, some fruit

o   Bonide Fungonil RTS (different from other Fungonil products, which contain chlorothalonil)

o   Bonide Infuse

o   Fertilome Liquid Systemic

·         Tebuconazole – systemic; ornamentals only

o   Bayer Advanced Disease Control

o   Bonide Rx Systemic

·         Triflorine – systemic; ornamentals only

o   Ortho Rose Pride Disease Control

Late in the growing season, powdery mildew fungi produce small, black overwintering structures that can serve as inoculum the following year.

Disease-Resistant Knockout Rose Susceptible to Rose Rosette Disease

Disease-Resistant Knockout Rose Susceptible to Rose Rosette Disease

Remember when Knockout rose was the answer to our rose disease problems?  Suddenly, they were everywhere.  I, too, am guilty of mass plantings in both residential and commercial settings.  Then came rose rosette disease; Knockout was susceptible.

During the past 10 years or so, rose growers have struggled with this previously mysterious affliction.  The disease was first described in the 1930’s but it began to spread through our area in the 1980’s.  With the introduction of the Knockout series, roses became popular again.  Consequently, with more roses came more extensive disease spread. 

Rose rosette disease was first diagnosed on Knockout rose in Kentucky in 2009.  Since then, the UK Plant Disease Diagnostic Lab has witnessed a rapid increase in incidence, especially with the disease-resistant Knockout.  This year seems consistent with that trend.

Symptoms:  Disease symptoms vary with rose cultivar, but combinations of all or some of the most distinct symptoms are used for diagnosis. 

·         Stem bunching or clustering, witch’s broom

·         Elongated and/or thickened canes

·         Bright red leaves and stems

·         Excessive thorniness, small red or brown thorns

·         Distorted or aborted flowers

·         Under-developed or narrow leaves and/ or distorted canes

·         Dead or dying canes, yellow or brown foliage, dwarfing or stunting

Causal Agent:  Causal agent of rose rosette disease is a virus classified in the newly described genus Emaravirus. It is systemic and occupies all tissue with a plant, although only some plant parts may be symptomatic.  This pathogen is not spread by pruners or other mechanical means like some common viruses, but it is readily transferred onto rootstocks through grafts.  Due to high demand of Knockout roses, mass production has led to spread of diseaed roses throughout industry. 

In the landscape or garden center, disease is spread by a small mite, the rose leaf curl mite that resides in axillary buds.  Mites are transported on insects or by wind currents for up to 100 yards.

Disease Management:  Viruses become systemic in plants, and there is no cure for infected roses.  Thus, diseased plants must be destroyed so that neither the mite nor the virus is spread to healthy plants.  Mite control is difficult if not impossible, and growers may risk destroying beneficial organisms with excessive insecticide or miticide use.  Prevention is the best control for rose rosette disease.  Growers and homeowners should carefully inspect plants before purchase.  If symptoms are present, avoid purchasing roses from that supplier.  As aforementioned, destroy infected plants immediately and never hold them over in an attempt to cure.  Wild multiflora roses are ideal hosts for both the mite and the virus, so consider destroying nearby wild roses as well. 

What's this spot on my rose? Downy mildew showing up on rose this spring.

There’s a new disease in town.  Downy mildew of rose is showing up across the state.  It has been several years since downy mildew has been diagnosed on rose in KY, but we have already seen multiple cases this spring.  During the past weeks, there have been reports of an “incurable black spot” that is “unresponsive to fungicides.”

The downy mildew pathogen is a water mold, not a true fungus.  Thus, humid greenhouse conditions are ideal for disease spread.  So far, all reports originate from greenhouse-grown roses.  Additionally, retail centers who purchased finished roses indicated that roses arrived with early stages of the disease. 

Symptoms of downy mildew occur primarily on young apical leaves.  Initial symptoms begin as purplish-red irregular spots (photo below).  Rapid spread occurs with high moisture and reduced air circulation.  Within days, lesions coalesce, resulting in leaf yellowing and/or leaf drop.  The fuzzy sporulation of rose downy mildew occurs on the lower leaf surface, but it is typically very sparse and difficult to see.  This season, infection has been diagnosed on hybrid tea roses and some varieties of Knockout rose. 

Downy mildew on rose begins with irregularly-shaped lesions that are purple to reddish in color.

Roses are susceptible to a variety of leaf spots (photo below).  Black spot, the most common foliar disease of rose, begins as circular spots with feathery margins.  While hybrid tea roses are extremely susceptible to black spot, Knockout roses are mostly resistant.  Cercospora leaf spot, common on many shrub roses, produces circular lesions with purplish red edges and light gray or tan centers.  Both of these leaf spots can be confused with early symptoms of downy mildew.

Leaf spots of rose can appear similar.  Black spot lesions are circular with feathery edges (left, photo by J. Hartman).  Cercospora spots have dark halos with light centers (right, photo by A. Windham).  Downy mildew infections cause blotches, not spots (center, photo by D. Hull).

Management of downy mildew requires a combination of cultural and fungicidal controls.  This first step requires reduction of humidity.  Greenhouses should be well ventilated.  Nursery and greenhouse plants should be spaced for sufficient air circulation to promote leaf drying.  Drip irrigation can greatly reduce leaf wetness, as well.  Growers should practice good sanitation, removing fallen leaves and pruning away diseased plant parts, as the pathogen can survive for several weeks on debris.  Fungicides registered for use in commercial greenhouses and nurseries include Aliette, Banol, Segway, Stature, and Subdue MAXX.  Homeowners may apply copper + mancozeb.  Refer to labels for rates and application intervals.