Powdery Mildew in the Greenhouse

Dr. Sharon M. Douglas Department of Plant Pathology and Ecology The Connecticut Agricultural Experiment Station 123 Huntington Street, P. O. Box 1106 New Haven, CT 06504

Phone: (203) 974-8601 Fax: (203) 974-8502 Founded in 1875 Email: [email protected] Putting science to work for society Website: www.ct.gov/caes

POWDERY MILDEW IN THE GREENHOUSE

Powdery mildew is one of the most common fluffy and white to sparse and gray (Figures and widely distributed disease of plants in 1-3). greenhouse production. This disease is responsible for significant economic losses in many greenhouse floricultural (e.g., roses, violas, African daisy, zinnias) and vegetable (e.g., tomatoes, cucumbers) crops. More recently, powdery mildew has been implicated as a serious threat to poinsettia production. Although infections usually do not result in plant death, they reduce crop aesthetics and value. With tomato, infections can significantly reduce fruit production. In the greenhouse, powdery mildew tends to be more problematic in the spring and fall when day-night temperatures Figure 1. Powdery mildew of torenia. favor high relative humidity (RH) but it can develop at any time during any production Powdery mildew fungi usually attack young cycle. An understanding of the disease developing shoots, foliage, stems, and cycle, organisms involved, and factors that flowers, but can also colonize mature favor disease development will contribute to tissues. Symptoms often first appear on the successful management of this disease. upper leaf surface, but can also develop on lower leaf surfaces. Early symptoms vary SYMPTOMS: and can appear as irregular, chlorotic, or Powdery mildews are easily recognized by purple areas, or as necrotic lesions, all of the white, powdery growth of the fungus on which are followed by the typical white, infected portions of the plant host. The powdery appearance. Some infected leaves powdery appearance results from the may shrivel, brown, and drop prematurely. superficial growth of the fungus as thread- Other symptoms include atypical scab-like like strands (hyphae) over the plant surface lesions, witches’-brooms, twisting and and the production of chains of spores distortion of newly emerging shoots, (conidia). Colonies vary in appearance from premature leaf coloration and drop, slowed

or stunted growth, and leaf rolling. In rare CAUSAL ORGANISMS AND but extreme situations, heavy infections DISEASE DEVELOPMENT: cause plant death. Although the symptoms of disease are similar, the fungi responsible for powdery mildew fall into a number of different genera. The most common genera include Erysiphe, Golovinomyces, Phyllactinia, and Podosphaera. These fungi are all obligate parasites that require living hosts to complete their life cycles so they readily infect healthy, vigorous plants. Some powdery mildew fungi have broad host ranges whereas others are fairly host- specific. For example, Podosphaera macularis can infect many hosts such as

coreopsis, geranium, heuchera, delphinium, Figure 2. Powdery mildew of gerbera daisy. phlox, and potentilla, whereas Sphaerotheca sparsa is limited to asclepias. It is important Although diagnosis of powdery mildew is to know the identity of the fungus in order to not difficult, symptoms often escape early determine the potential for spread to other detection if plants are not periodically crops that might be present in the monitored, since symptoms can first develop greenhouse. on lower or middle leaves. The time delay from when infections begin and when Powdery mildew fungi have fairly simple disease is detected helps explain reports of life cycles on most plants. Spores (conidia) sudden “explosions” of disease. This can are produced in chains on stalks occur when the percentage of infected leaves (conidiophores) (Figure 4). Conidia are increases from 10% to 70% in one week. “powdery” and are readily disseminated by air currents.

Figure 4. Chains of powdery mildew conidia on Figure 3. Powdery mildew colonies on a conidiophores growing the surface of a leaf cucumber transplant. (arrows).

Powdery Mildew in the Greenhouse S. M. Douglas 2 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

After the conidia land on the plant surface, Development of powdery mildew in the they germinate, penetrate the tissues, and greenhouse is influenced by many send food-absorbing projections (haustoria) environmental factors including into the epidermal cells. Thread-like strands temperature, RH, light, and air circulation. of the fungus (hyphae) then grow over the Unfortunately, greenhouses usually provide surface of the infected plant part and optimum levels for all of these conditions. eventually produce more conidiophores and Optimum conditions include moderate conidia. The time from when conidia land temperatures (68-86° F), high humidities to the production of new conidia can be as (>95% RH), and fairly low light intensities short as 72 hours, but is more commonly 5-7 or shade. However, these requirements vary days. Powdery mildew conidia are unique with the specific powdery mildew fungus. since, unlike most fungal spores, they do not There is an inverse relationship between require free moisture (e.g., guttation, dew, temperature and RH which influences both rain, overhead irrigation) on plant surfaces the production and spread of powdery in order to penetrate and infect. mildew conidia. As temperatures fall at night, RH increases. High RH stimulates In greenhouses, powdery mildews usually conidia to germinate and also encourages the survive between crops as hyphae or fungal production of chains of conidia in existing strands in living crop plants or in weedy infections. In the morning after sunrise, the hosts. Under certain circumstances, some temperatures warm and RH levels fall. powdery mildew fungi produce small, black, These conditions help to dry the chains of pepper-like resting structures called conidia. Since conidia function as the chasmothecia (formerly called cleistothecia) primary means for new infections in the (Figure 5). These structures allow the greenhouse, air movement and circulation in fungus to survive in the absence of a suitable the house are very important for host. However, the role of these resistant development and spread of disease. Dry, structures is probably insignificant in “powdery” conidia are easily dislodged and greenhouse situations since continuous disseminated by air movement from grower cropping provides a constant source of activities in the house as well as by opening living hosts. and closing doors.

STRATEGIES FOR DISEASE MANAGEMENT: Although chemical control continues to be a key component of management of powdery mildew in the greenhouse, other strategies complement and enhance control efforts. 1. Culture and Pathogen Identification-  Maintain adequate plant spacing to reduce RH levels in the plant canopy. This also helps to obtain good coverage with fungicide sprays.

 Maintain RH levels below ~ 93% by Figure 5. Chasmothecia of different ages of properly timed venting and heating. maturity, from immature yellow to mature dark brown or black.

Powdery Mildew in the Greenhouse S. M. Douglas 3 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

 Identify the particular powdery mildew “Bingo Deep Purple” are highly resistant fungus in order to anticipate the potential to some powdery mildews. for spread to other plants in the house. 4. Chemical-  Syringing or applying water directly to Control with chemicals is targeted at leaves of some greenhouse crops eradication of existing infections and discourages germination of conidia and protection of healthy tissues. Once disease helps to wash conidia off leaf surfaces. is detected, the first sprays should be aimed This procedure works for some crops at eradication. These are usually followed provided other types of foliar diseases by sprays for protection. The efficacy of favored by leaf wetness are not common specific compounds can vary significantly problems for that crop. with the particular powdery mildew fungus 2. Monitoring and Sanitation- and host, so knowledge of the host-pathogen  Carefully examine and inspect new combination is helpful. Attention to spray cuttings, seedlings, and plugs upon delivery and coverage is also very arrival. Never use diseased plant important. Other factors for consideration material. are fungicide classes (MOA--FRAC Code)  Scout for disease on a regular schedule for resistance management, REI, to identify outbreaks before they become environmental parameters (e.g., T, RH), widespread. This typically involves compatibility, residue, and stage of the crop examining one out of 30 plants each production cycle. week. It is helpful to concentrate on the  Eradication sprays should be applied as middle and lower leaves since infections soon as symptoms are first observed often start in these leaves. Once disease since early control is critical!!! is detected, examine one out of 10 plants  Monitor and rotate the types of every week. Continue with this schedule compounds used to avoid development until plants are free of disease for at least of fungicide resistance in the powdery three weeks. Thereafter, resume weekly mildew population. The diversity of scouting of one plant out of 30. products currently registered and  All diseased tissues should be removed effective for greenhouse use makes as soon as they are detected and fungicide resistance management much immediately placed in a plastic bag to easier than in the past. Since pesticide avoid carrying infected material through registrations vary with state, check with the house. the appropriate agency and consult the  All production areas should be label before applying any pesticide. thoroughly cleaned and plant debris  General categories of compounds removed between crops and production – Biologicals: cycles. This includes removing all  Bacillus subtilis (Cease, weeds in and around the greenhouse. Rhapsody, Serenade) 3. Resistance-  Trichoderma harzianum Rifai  Genetic resistance is very effective for strain KRL-AG2 (PlantShield) powdery mildew control but it is – Systemics: unfortunately of limited availability for  Strobilurins [QoI] (Compass O, most floricultural crops. For example, Cygnus, Insignia, Heritage) the “Profusion” series of Zinnia elegans  DMIs (Terraguard, Eagle, Hoist, is resistant to powdery mildew and Strike) pansy cultivars “Delta Pure Rose” and

Powdery Mildew in the Greenhouse S. M. Douglas 4 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)

 Thiophanates (Cleary’s 3336, OHP 6672)  Carbamate and Strobilurin (Pageant) – Contacts (*Biorationals):  Bicarbonates (Milstop, Kaligreen) *  Coppers (Camelot, Kocide, Phyton 27)  Hydrogen dioxide (ZeroTol, Oxidate)  Sulfur (Microthiol Disperss)  Oils: Horticultural & Neem (Ultra-Fine Oil, Triact) *  Soaps (Insecticide Soap) *

August 2012 (revised)

Powdery Mildew in the Greenhouse S. M. Douglas 5 The Connecticut Agricultural Experiment Station (www.ct.gov/caes)