University of FloridaSolutions for Your Life

Download PDF
Publication #PP257

Powdery Mildew of Gerbera Daisy1

Catalina Moyer and Natalia A. Peres2

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 lower plant quality contribute to economic losses due to this plant disease (Figure 1).

Figure 1. 

Gerbera daisy plant severely affected by powdery mildew


[Click thumbnail to enlarge.]

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


[Click thumbnail to enlarge.]

Symptoms

Powdery mildew is easy to identify since noticeable white spots or white patches appear on the upper and lower surfaces of the leaves (Figure 3A). These spots gradually enlarge to form a white, powder-like mat that 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


[Click thumbnail to enlarge.]

Figure 4. 

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


[Click thumbnail to enlarge.]

Disease Development and Spread

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

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) that enlarges and penetrates the leaf epidermis and lives on the host plant. Other specific fungal structures develop 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 leaf surfaces. 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), and Spectro (chlorothalonil + thiophanate-methyl). Additional fungicides for control of powdery mildew on ornamentals are listed in Table 1.

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

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 pathogens. These products are generally narrow-spectrum, and they decompose quickly. Because 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, fruit, and vegetable 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 several trade names and 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). Kaligreen, Armicarb and Milstop, in particular, are potassium bicarbonate fungicide prodcuts that have proven effective to reduce powdery mildew of gerbera daisy on experiments conducted in Florida.

Disease Management Summary

Powdery mildew inoculum is carried by air. When conditions are favorable, this inoculum can spread rapidly 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.)

Tables

Table 1. 

Fungicides registered to control powdery mildew on ornamentals sorted by their active ingredient.

Trade name

Active ingredient

Fungicide groupa

Heritage

azoxystrobin

11

Nufarm TM + CTN SPC 66.6 WDG

Spectro 90 WDG

chlorothalonil + thiophanate-methyl

M5 + 1

Champ DP Dry Prill

Champ Formula 2 Flowable

Champ WG

Kentan DF

Nu Cop 50 WP,

Nu-Cop 3 L,

Nu-Cop 50 DF,

Nu-Cop HB

copper hydroxide

M1

Badge SC

Badge X2

copper hydroxide + copper oxychloride

M1

COC DF,

COC WP

copper oxychloride

M1

Nordox,

Nordox 75 WG

cuprous oxide

M1

Cuprofix Ultra 40 Disperss

copper sulfate

M1

Tourney

metconazole

3

Eagle 20EW,

Prokoz Hoist

Quali-Pro Myclobutanil 20 EW

Rally 40WSP

myclobutanil

3

Affirm WDG

polyoxin D zinc salt

19

Fosphite Fungicide

Nutrol

Rampart

potassium phosphite

33

Sanction

potassium phosphate + sulfur

33 + M2

Amtide propiconazole 41.8% EC

Banner Maxx

Nufarm propiconazole SPC 14.3

Procon Z

Prokoz Fathom 14.3 MEC

Propensity 1.3ME

Propi-Star EC

Quali-Pro Propiconazole 14.3

propiconazole

3

Insignia

pyraclostrobin

11

Cosavet DF

Micro Sulf

Microfine Sulfur

Microthiol Disperss

Sulfur 90W (Drexel)

Yellow Jacket Dusting Sulfur

Yellow Jacket Wettable Sulfur

sulfur

M2

Clearscape

Monsoon Turf

Quali-Pro Tebuconazole 3.6 F

Torque Fungicide

tebuconazole

3

3336 F

3336 WP

Nufarm T-methyl SPC

Prokoz Transom

thiophanate-methyl

1

Compass

trifloxystrobin

11

Terraguard 50W

triflumizole

3

aFungicide group (FRAC Code): Numbers (1-37) and letters (M) are used to distinguish the fungicide mode of action groups. All fungicides within the same group (with same number or letter) indicate same active ingredient or similar mode of action. M = Multi site inhibitors which have low fungicide resistance risk. Fungicides with different chemical groups should be rotated to avoid the development of fungicide resistance.

Table 2. 

Biopesticides registered to control powdery mildew on ornamentals.

Trade name

Active ingredient

Cease

Rhapsody

Bacillus subtilis strain QST 713

Sporan EC

clove oil + rosemary oil + thyme oil

Trilogy

neem oil (clarified hydrophobic extract)

Saf-T-Side

petroleum oil

Armicarb 100 Fungicide

Kaligreen

Milstop

potassium bicarbonate

Sil-Matrix

potassium silicate

Regalia

Extract of Reynoutria sachalinensis

Prev-Am

sodium tetraborohydrate decahydrate

Note: Not all legally available products sold in Florida are listed. For such list contact the Florida Department of Agriculture. Make sure to read a current product label before applying any chemicals. If the label does not specifically prohibit greenhouse use, the material can be used in the greenhouse.

Footnotes

1.

This document is PP257, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date July 2008. Revised May 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Catalina Moyer, biological scientist, and Natalia A. Peres, associate professor, Plant Pathology Department, Gulf coast REC-Wimauma, FL, 33598, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label. Use pesticides safely. Read and follow directions on the manufacturer's label.


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 UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.