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Publication #PP311

Management of Powdery Mildew in Beans1

Qingren Wang, Shouan Zhang, and Teresa Olczyk2

Powdery mildew, caused by the fungal organism Erysiphe polygoni, is one of the most commonly occurring diseases on many types of beans. Green bean, pole bean, long bean, Italian bean, and snow pea crops are all susceptible to powdery mildew in tropical and subtropical climates. Although the causal organism rarely causes extensive damage, significant yield losses (up to 69%) were reported in Columbia County when infection occurred in dry beans prior to flowering (Schwarts et al. 2005). Accurately identifying this disease and immediately taking action for control are critical to effectively prevent spread of powdery mildew in order to reduce significant losses of yield and quality.

Symptoms

Powdery mildew can affect all above-ground parts of bean plants. Initial symptoms appear as small and white talcum-like spots (Figure 1), which most commonly are seen on the upper surface of leaves. These spots increase in size and run together to form a whitish, powdery growth, gradually spread over a large area of the leaves (Figures 2, 3, and 4), and can spread even farther to the stems. As the symptoms developed, infected leaves may gradually curl downward, pale yellow or brown (Figure 5), die, and fall off. Under severe conditions, the entire leaves and plants could be covered by white cottony mycelial growth of the fungus (Figure 6). Symptoms on infected leaves may vary with bean varieties, but powdery mildew may cause the leaves to be twisted, buckled, or distorted. The powdery mildew fungus usually does not grow on bean pods except pea pods (Davis et al. n.d.). However, powdery mildew spots can develop on snap bean pods (Pohronezny and Stall n.d.). The development of powdery mildew symptoms is not often observed on pole bean in Miami-Dade County, but it is apparent on both Italian bean and long bean (dark green type) (Figures 7 and 8) grown under the same conditions. Severely infested plants may have reduced yields, shortened production periods, and even completely die (Fig. 8).

Figure 1. 

Early stage of powdery mildew development on Italian bean


Credit:

Qingren Wang


[Click thumbnail to enlarge.]

Figure 2. 

Powdery mildew development on pole bean


Credit:

Qingren Wang


[Click thumbnail to enlarge.]

Figure 3. 

Powdery mildew on snap bean


Credit:

Qingren Wang


[Click thumbnail to enlarge.]

Figure 4. 

Powdery mildew on long bean


Credit:

Qingren Wang


[Click thumbnail to enlarge.]

Figure 5. 

Severe development of powdery mildew on Italian bean


Credit:

Qingren Wang


[Click thumbnail to enlarge.]

Figure 6. 

Development of powdery mildew on pole bean plants without full sunlight


Credit:

Qingren Wang


[Click thumbnail to enlarge.]

Figure 7. 

Powdery mildew development on Italian bean pod


Credit:

Qingren Wang


[Click thumbnail to enlarge.]

Figure 8. 

Powdery mildew development on long bean pods


Credit:

Qingren Wang


[Click thumbnail to enlarge.]

Infection and Spread

The fungus of powdery mildew grows as thin layers of mycelium (fungal tissue) on the surface of the affected leaves. Spores, which are the primary means of dispersal, make up the bulk of the visible white, powdery growth. Powdery mildew spores can be easily carried by wind to new hosts. The spores can germinate and infect beans in the absence of free water. Powdery mildew growth generally does not require moist conditions (Davis et al. n.d.); however, increased humidity can increase the severity of the disease (Pohronezny and Stall n.d.). Moderate temperatures (60°F to 80°F) and shady conditions generally are the most favorable environmental factors for the development of powdery mildew (Fig. 6). Strong and direct sunlight with high temperatures (over 90°F) can noticeably reduce the incidence of the disease.

Management Practices

Preventive Practices: Prevention is the optimal method for controlling powdery mildew. Growing resistant varieties or cultivars of beans wherever available, planting in full sun, and following good cultural practices can adequately control powdery mildew in most cases.

Cultural Practices: Plant in sunny areas as much as possible and provide good air circulation. Overhead watering may help reduce powdery mildew, because spores might be washed off the plants. However, such a measure may contribute to other pest management concerns.

Fungicide Application: In vegetable fields, the least-toxic fungicides may be used to prevent, protect, or eradicate powdery mildew. Apply protective fungicides to highly susceptible varieties before the disease appears, and utilize eradicative fungicides at the earliest signs of the disease. Once powdery mildew growth becomes prevalent, control with any fungicide can be difficult. Below are recommendations for fungicides for control of powdery mildew on beans. Make sure to follow the application instructions on the labels.

References

Compendium of Bean Diseases. Second Edition. Eds. H. F. Schwarts, J. R. Stedman, R. Hall, and R. L. Forster. The American Phytopathological Society, 2005.

R.M. Davis, W.D. Gubler, S.T. Koike, M.L. Flint, and P.N. Galin, Plant Pathology, UC Statewide IPM Program, University of California, Davis, CA 95616.

K. Pohronezny and W. M. Stall. Powdery Mildew of Vegetables, Plant Pathology Fact Sheet, PP-14. Florida Cooperative Extension Service / Institute of Food and Agricultural Science/ University of Florida, Gainesville, FL 32611.

Tables

Table 1. 

Fungicide recommendations for powdery mildew control on beans

Fungicide

(active ingredients)

Fungicide Group

REI* (hours)

OMRI-listed**

Remarks

Actinogrow,

Actinovate

(Streptomyces lydicus WYEC 108)

 

1 h

Yes

 

Armicarb 100 Fungicide

(potassium bicarbonate)

 

4 h

No

 

Kaligreen Fungicide (potassium bicarbonate)

 

4 h

Yes

 

Milstop (potassium bicarbonate)

 

1 h

Yes

 

Confine Extra,

Kphite 7LP AG

(phosphorous acid)

33

4 h

No

 

Double Nickel LC

(Bacillus amyloliquefaciens strain D747)

 

4 h

Yes

 

Fungi-Phite,

Fosphite Fungicide,

Rampart

(potassium phosphite)

33

4 h

No

 

Oxidate

(hydrogen dioxide)

 

1 h (for enclosed areas)

No

 

Regalia SC

(extract of Reynoutria sachalinensis)

P

4 h

Yes

 

Sonata,

Sonata Fungicide

(Bacillus pumilus strain QST2808)

 

4 h

Yes

 

Sporatec

(clove oil, rosemary oil, thyme oil)

 

0

Yes

 

Cueva

(copper occanoate)

M1

4 h

Yes

 

Kumulus DF Fungicide-Acaricide, Micro Sulf, Microfine Sulfur, Microthiol Disperss, Sulfur 90W, Wettable Sulfur, Yellow Jacket Dusting Sulfur, Yellow Jacket Wettable Sulfur (sulfur)

M2

24 h

Yes—for Kumulus, Microfine Sulfur, MicroSulf, Microthiol Disperss

Do not apply during periods of warm weather to avoid phytotoxicity

Top Cop with Sulfur (basic copper sulfate + sulfur)

M1 & M2

24 h

No

 

Fontelis,

Vertisan (penthiopyrad)

7

12 h

No

 

Headline,

Headline SC (pyraclostrobin)

11

12 h

No

 

Priaxor

(fluxapyroxad + pyraclostrobin)

7 & 11

12 h

No

 

*REI: Restricted Entry Interval

**OMRI: Organic Materials Review Institute. OMRI-approved products can be used for organic production.

Footnotes

1.

This document is PP311, one of a series of the Plant Pathology Department, UF/IFAS Extension. Original publication date June 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Qingren Wang, commercial vegetable Extension agent and pesticide trainer, UF/IFAS Miami-Dade County Extension; Shouan Zhang, associate professor, UF/IFAS TREC-Homestead and Plant Pathology Department; and Teresa Olczyk, director, UF/IFAS Miami-Dade County Extension.


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.