• Topics: Plant Pathology  | Gerbera  |  Nursery and Ornamental Plant Diseases  |  Peres, Natalia A 

Powdery Mildew of Gerbera Daisy¹

Introduction

Powdery mildew, caused by Erysiphe cichoracearum or Podosphaera fusca (Sphaerotheca fusca), is an important fungal disease of gerberas in Florida. This disease can affect all parts of the plant. Reduced plant growth and lesser plant quality contribute to economic losses due to this plant disease (Figure 1).

Figure 1. 

Gerbera daisy plant severely affected by powdery mildew

The fungus that causes powdery mildew is an obligate parasite; it can only infect living tissue. Powdery mildew reproduces prolifically by spores (conidia) (Figure 2), which can germinate in the absence of free water and are spread by air currents and water splash.

Figure 2. 

Microscopic view of powdery mildew conidia

Symptoms

Powdery mildew is easy to identify since noticeable white spots or white patches appear on the upper and lower surfaces of the leaf (Figure 3A). These spots gradually enlarge to form a white, powder-like mat, which can spread to flowers and stems (Figure 3B). The spots are a combination of body of the fungus – thread like (hyphae) (Figure 4A) -- and upright structures, the conidiophores, which bear the conidia (Figure 4B). Severely infected leaves turn pale yellow or brown and eventually die.

Figure 3. 

A) Early symptoms of powdery mildew on gerbera daisy leaves B) Advanced powdery mildew symptoms on gerbera daisy

Figure 4. 

A) Macroscopic view of conidiophores, conidia and hyphae on gerbera daisy leaf B) Microscopic view of conidiophore from gerbera daisy leaf

Disease Development

High humidity (80% - 90%) and moderate temperatures (68 - 82°F) contribute to the development of powdery mildew. Overcrowding and shading of plants will also favor development of this disease.

The powdery mildew fungus reproduces through sexual spores (ascospores), which serve primarily for survival of the fungus. It also reproduces through asexual spores (conidia), which are responsible for most of the spread of the disease.

The initial infection typically occurs when conidia from infected transplants are carried by air currents to other plants or plant parts. The conidia then germinate and produce an elongated structure (germ tube), which enlarges and penetrates the leaf epidermis to feed on the host plant. Other specific fungal structures are developed within the plant tissue to establish a biotrophic (symbiotic) relationship necessary to absorb nutrients. The germ tube continues to grow and spreads across the leaf surface, producing more hyphae and conidia.

Management Practices to Control Powdery Mildew

Cultural Control

Only plantlets that are free of powdery mildew should be used, or the plantlets should be disinfested before transplanting. In addition, plants should be grown in sunny areas with good air ventilation and without excessive fertilization. Overhead irrigation may reduce powdery mildew since it washes conidia from the surfaces of leaves. However, overhead irrigation may contribute to other disease problems, such as crown rot

Chemical Control

In Florida, labeled fungicides to control powdery mildew in gerberas include the following: Compass (trifloxystrobin), Eagle (myclobutanil), Heritage (azoxystrobin), Milstop (potassium bicarbonate), Spectro (chlorothalonil + thiophanate-methyl) and Strike (triadimefon). Additional fungicides to control powdery mildew in ornamentals are listed in Table 1.

However, chemical control may not always be completely effective since the fungus may be resistant to some chemicals. Powdery mildew goes through many short cycles per season, and this process increases potential for development of resistance, especially when only systemic fungicides are used to control this disease. Resistance of powdery mildew has already been confirmed with the fungicides benomyl (Benlate) and triadimefon (Bayleton).

Biorational or Biofungicidal Control

Biofungicides are naturally based microbial or biochemical products derived from animals, plants, bacteria, and certain minerals. Biofungicides can affect fungal organisms directly or may stimulate the plant's own defense against the organisms. These products are generally narrow-spectrum, and they decompose quickly. As a result of these characteristics, biofungicides are considered to have little potential for negative impact on the environment. Biofungicides that have been effective for the control of powdery mildew include biological control agents (BCAs), oils, phosphorous acid, and potassium bicarbonate.

Biological Control Agents: BCAs are organisms that suppress pests or plant pathogens. BCAs that have been used successfully for the control of powdery mildew include the following: Ampelomyces quisqualis (AQ10), Tilletiopsis spp, Pseudozyma flocculosa (Sporodex, Plant Products Co., Brampton, Ontario, Canada) and Bacillus subtilis QST 713 (Serenade® or Rhapsody®, AgraQuest, Inc. Davis, California). The major impediment to BCA effectiveness is their requirement for high humidity.

Oils: Petroleum or plant oils have not only been effective in controlling powdery mildew, but also have been useful in reducing the development of resistance to fungicides. Reduction of the severity of powdery mildew through use of oils has been reported in other ornamental crops, as well, including roses, apples, cherries, cucurbits, and grapes. In some cases, the efficacy of oils to reduce powdery mildew is comparable to or even superior to results obtained through use of standard fungicides.

Phosphorous Acid: Phosphorous acid (H₃PO₃) is the active ingredient in phosphonate, used in agriculture for disease control. However, H₃PO₃ is not a nutritional source of P for plants. Nonetheless, H₃PO₃ is often confused with phosphoric acid (H₄PO₄) or phosphates, which are effective nutritional sources of P.

Phosphite-based products are marketed under the following trade names: AGRI-FOS, BioPhos or Vital (Agrisel USA, Inc., Brookfield, Connecticut ), Aliette or Fosetyl-Al (Bayer CropScience, North Carolina), K-phite (Plant Food Systems, Inc., Zellwood, Florida), ProPhyt (Luxembourg Industries Ltda., Tel Aviv, Israel), and Resist 57 (Actagro LLC., Biola, California). These products have all been found effective in reducing the severity of powdery mildew on gerbera daisy and on other crops, such as grapes and muskmelons.

Potassium Bicarbonate: Some potassium-bicarbonate-based products are labeled as fungicides and have been approved by the U.S. Environmental Protection Agency (EPA). Armicarb 100 (Helena Chemical Company, Collierville, Tennessee), Kaligreen (AgBio Inc., Westminster, Colorado), MilStop (BioWorks, Inc. Fairport, New York), and Remedy (Bonide Products, INC., Oriskany, New York) are trade names for potassium-bicarbonate-based fungicides labeled for control of powdery mildew in conventional systems. Kaligreen, Armicarb and Milstop, in particular, have been used successfully to reduce powdery mildew of gerbera daisy.

Disease Management Summary

Powdery mildew inoculum is carried by air. When conditions are favorable, this inoculum can rapidly spread over gerbera plants, creating a severe epidemic. When disease pressure is low, the biorational measures discussed above can provide a satisfactory level of control. When disease pressure is moderate or high, chemical measures may be more effective. The best strategy for managing fungicide resistance is to use various products with different modes of action. Fungicide mode-of-action groups are listed in Table 1. (All fungicides within the same group -- with same number or letter -- indicate the same active ingredient or similar mode of action.)

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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