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© 2000 by Plant Health Progress.
Accepted for publication 19 July 2000. Published 28 July 2000.


Managing Powdery Mildew of Cherry in Washington Orchards and Nurseries with Spray Oils


G. G. Grove, Associate Professor of Plant Pathology, Washington State University Irrigated Agriculture Research and Extension Center, Prosser, WA, 99350; R. J. Boal and L. H. Bennett, Research Associates, Washington State University Tree Fruit Research and Extension Center, Wenatchee, WA, USA 98801.


Corresponding author: Gary G. Grove. grove@wsu.edu

Grove, G. G., Boal, R. J., and Bennett, L. H. Managing powdery mildew of cherry in Washington orchards and nurseries with spray oils. Online. Plant Health Progress doi:10.1094/PHP-2000-0728-01-RS.



Abstract

Powdery mildew of sweet cherry is the most serious disease in irrigated orchards and nurseries of Washington. The fungus infects foliage and fruit, but fruit infections are of primary concern due to the potential for rejection of entire crops. Development of resistance to demethylation inhibiting (DMI) fungicides in the mid-1990s increased the difficulty in controlling this disease. Orchard mildew management programs that utilize oils, DMI and strobilurin fungicides have been developed for use in eastern Washington. A growth stage (phenology)/calendar-based program provides excellent disease control, utilizes three fungicidal modes of action, and minimizes oil-induced fruit and foliar phytotoxicity by limiting oil use to no later than the pit hardening stage. A second approach that utilizes oils in a temperature-based disease forecasting system provides excellent disease control with fewer fungicide applications.

Powdery mildew of sweet cherry (Prunus avium L.), which is caused by Podosphaera clandestina (Wall.:Fr) Lev. (2), is a chronic disease in irrigated orchards and nurseries of Washington (1, 2). The disease is common on foliage (Fig. 1), which serve as sources of inoculum for fruit infections (Fig. 2). In 1986, 1987, 1993, and 1995, rain showers occurred in mid-June, resulting in high incidences of fruit infection. Infection of fruit by P. clandestina can result in epidermal scarring and mycelial growth on the fruit surface. Many Washington producers have reported the rejection of entire crops destined for fresh-market sale due to fruit infection by P. clandestina.


Fig. 1. Foliar symptoms of powdery mildew of sweet cherry, caused by Podosphaera clandestina. Foliar mildew colonies serve as an inoculum source for fruit infections that can occur during fruit development (Click image to download larger view). Fig. 2. Sweet cherry fruit infected by P. clandestina. Scarring and the presence of superficial mycelium are characteristic symptoms of the disease (Click image to download larger view).


P. clandestina survives as cleistothecia on the orchard floor, tree crotches, and bark crevices (2). Rains or impact sprinkler irrigation from April to early- or mid-May promote ascospore release and primary infection of foliage. Mildew colonies originating from these infections are usually few in number, inconspicuous, and generally appear in the orchard during late-April to mid-May. Secondary foliar infections, which are more obvious, occur over the next 6 to 8 weeks preceding harvest and persist after harvest. In the absence of spring rains, primary infection can become a site-specific process due to differing irrigation practices and timings. Primary infection requires thorough wetting of tree trunks and temperatures of > 10°C (2). Most Washington growers do not have on-site weather monitoring equipment capable of measuring temperature, rainfall, and leaf wetness, which makes identification of primary infection periods difficult. For these reasons, early season suppression of primary mildew colonies using multi-site mode-of-action fungicides may provide an alternate means of managing the disease.

Beginning in 1988, the demethylation-inhibiting (DMI) fungicides were routinely used to manage the disease in orchards and nurseries and eventually became the sole chemical components of powdery mildew management programs in Washington. Up to eight applications at 10- to 14-day intervals were routinely made when disease pressure was severe. The first reports of DMI-related control failures occurred in the mid-1990s (Grove, unpublished data). With the advent of powdery mildew resistance to DMI fungicides (12, 18) and the potential for resistance to strobilurin fungicides (19), it became apparent that one or more classes of multi-site compounds required investigation as mildew control agents and as tools for managing resistance to DMI and strobilurin fungicides. Prior to the 1990s, sulfur compounds were the only multi-site compounds available for use in cherry powdery mildew management programs. Sulfur compounds have a short residual activity, are effective over a limited temperature range, pose significant phytotoxicity risks, and may interfere with arthropod/mite integrated pest management (IPM) programs in Eastern Washington.

The purposes of these studies were to investigate spray oils for their efficacy against P. clandestina on sweet cherry and to integrate promising compounds into powdery mildew and fungicide resistance management programs in Eastern Washington.


Phenology/Calendar Based Programs

JMS Stylet Oil (JMS Flower Farms, Vero Beach, FL) was the principal oil fungicide used in these studies. Other fungicides utilized included myclobutanil (Rally 40 W, Rohm and Haas Company, Philadephia, PA); kresoxim-methyl (Sovran 50 WG, BASF, Research Triangle Park, NC); azoxystrobin (Abound, Zeneca Agrochemicals, Wilmington, DE); trifloxystrobin (Flint 50WG, Novartis Crop Protection, Greensboro, NC); fenarimol (Rubigan, DowAgro Sciences, Indianapolis, IN); M-Pede (Mycogen Co., San Diego, CA 92121); Orchex 796 (Exxon Co., Houston, TX); Microthiol 80W flowable sulfur (Elf Atochem, Philadelphia, PA); propiconazole (Orbit, Novartis Crop Protection, Greensboro, NC); and triflumizole (Procure 50WS, Uniroyal Chemicals, Middlebury, CT).



Fig. 3. Fruit phytotoxicity resulting from four sequential applications of 2% JMS Stylet Oil during fruit development. Damage to fruit includes pitting, uneven coloration, and loss of fruit luster (Click image to download larger view).

Experiments were conducted in a 0.81 ha sweet cherry (cv. Bing) orchard located about 5 km south of Orondo, WA. The orchard was comprised of 9-yr-old trees on Mazzard rootstock. Tree spacing was 6.1 m between trees within a row and 7.0 m between rows. Treatments consisted of six single-tree replications arranged in a randomized complete block design. Fungicide rates were calculated on the spray volume of 3,785 L/ha. The treatments and rates applied in 1997 and 1998 are presented in Tables 1 and 2, respectively. Fungicides were applied to drip using a handgun sprayer at 89.4 kg/cm2. In 1997, the initial applications were made about 2 weeks after full bloom. Additional sprays were made at 2-week intervals over the subsequent 6 weeks. In 1998, the initial application was made about 2 weeks after full bloom but JMS Stylet Oil was used only in the first and/or second sprays; the subsequent 2-3 sprays were comprised of DMI and/or strobilurin fungicides. About 4 and 2 weeks prior to harvest in 1997, disease severity was evaluated by randomly selecting five terminal shoots per replication and counting the number of mildew colonies on five leaves beginning at the first fully expanded leaf from the shoot apex. Because of difficulties of discerning individual colonies during the latter stages of leaf colonization, a different method was used to assess disease severity in 1998 and 1999. Foliar mildew severity was determined about 1 week prior to harvest by selecting 10 terminal shoots on each tree and determining the percent leaf area colonized on each of ten leaves beginning at the first fully expanded leaf beneath the shoot apex. Therefore, the foliar disease severity results of 1997 and 1998/1999 cannot be directly compared. Fruit phytotoxicity (Fig. 3) was determined by randomly harvesting 100 fruit from each replication and visually assessing damage to the fruit surface and finish. Fruit with reduced luster and obvious epidermal pitting were considered damaged by oil. Mildew severity and fruit phytotoxicity data were subjected to analysis of variance and means separated with Fisher's Protected LSD test (P < 0.05).

Disease pressure was high during both years of the study. In 1997, powdery mildew in the untreated controls was 19.6 and 33.9 colonies/leaf at the first and second evaluations, respectively (Table 1). Disease severity ranged from 0.9 colonies/leaf in the myclobutanil treatment to 1.6 in the 38 L/ha Stylet Oil treatment at the first evaluation and from 0.1 in the 76 L/ha Stylet Oil treatments to 4.6 in the myclobutanil treatment at the second evaluation. Fruit damage was 0, 0, 4.8, and 10.3% in the untreated controls, myclobutanil, 38 L/ha and 76 L/ha Stylet Oil treatments, respectively.

Table 1. Effect of JMS Stylet Oil and myclobutanil on severity of powdery mildew of sweet cherry at Orondo, WA 1997.

Treatment1

Rate/ha    

Disease
severity
30 May
2,4

Disease
severity
30 June
2,4

Fruit
damage
3,4

Stylet Oil

38L

1.6 a4

1.0 a

4.8 b

Stylet Oil

76L

1.0 a

0.1 a

10.3 c

myclobutanil

140g ai

0.9 a

4.6 b

0 a

untreated

19.6 b

33.9 c

0 a

    

1 Sprays were applied according to host phenology from about 2 weeks after full bloom until 1 week prior to harvest. The interval between sprays was about 14 days.

2 Number of powdery mildew colonies per leaf.

3 Average percentage of damaged cherries in 100 fruits from each of four replications. Fruit with obvious pitting and/or lack of luster were considered damaged.

4 Means followed by the same letters are not significantly different according to Fisher’s Protected LSD test at P < 0.05.



Fig. 4. Foliar phytotoxicity on sweet cherry resulting from four sequential applications of JMS Stylet Oil. Characteristic oil damage includes chlorosis, spotting, and leaf curling. Severe damage may result in premature leaf abscission (Click image to download larger view).

In 1998, disease severity in the untreated control was 81.5% infected leaf area (Table 2) and ranged from 10.2% in the 38 L Stylet Oil treatment to 31.7% in the propiconazole treatment. There was no damage to foliage or fruit in the untreated control, propiconazole, or triflumizole treatments, but damage to fruit ranged from 0.1% in one of the Stylet Oil/azoxystrobin/propiconazole treatments to 8.5% in the 19 L Stylet Oil treatment. The exclusive use of oil from shuck fall to harvest resulted in damage to foliage (Fig. 4) during both years of the study.


Use of JMS Stylet Oil in Weather-based Fungicide Spray Programs

The utility of spray oils in weather-driven fungicide programs was investigated in 1999 in the orchard described above (Table 3). In most treatments, JMS Stylet Oil was applied at the first visible signs of disease, which were noted about 3 weeks after full bloom. At the time of the initial fungicide application, the secondary infection component of the Gubler-Thomas grape powdery mildew model (13) was initiated. This component identifies conditions as disease-conducive if > 6 hours of temperatures between 21.1 and 29.4°C occur during a 24- hr period (13). The risk index generated by the model is initiated when the temperature parameters are met on three consecutive days. Temperature, leaf wetness, relative humidity, and precipitation were continuously monitored using sensors connected to an Adcon (Adcon Telemetry, Santa Rosa, CA) A730MD weather monitoring system that was positioned near the center of the orchard. Subsequent DMI, sulfur, and/or strobilurin applications were made at intervals specified by the model, which called for two DMI or strobilurin sprays at 21-day intervals or three sprays of flowable sulfur or calcium polysulfide at 14-day intervals. For comparison, a calendar/phenology based "industry standard" spray approach, which included two strobilurin applications in alternation with two DMI applications, was included in the trial. Foliar mildew severity was determined about 1 week prior to harvest by selecting 10 terminal shoots on each tree and determining the percent leaf area colonized on each of 10 leaves beginning at the first fully expanded leaf beneath the shoot apex. Data were subjected to analysis of variance and means separated with Fisher's Protected LSD test (P < 0.05).

Table 2. Effect of JMS Stylet Oil, DMI, and strobilurin fungicides, and fungicide rotations on severity of powdery mildew of sweet cherry at Orondo, WA 1998.

Treatment

Rate/ha

Timing1

Leaf area diseased (%)

Fruit damage3 (%)

Stylet Oil +
 azoxystrobin +
 propiconazole

38L
1.17L ai
0.29L ai

SF, F
1C
2C

11.9 ab2

 

0.3 b

 

Stylet Oil +
 azoxystrobin +
 propiconazole

38L
1.17L ai
0.29Lai

SF
F, 1C
2C

15.8 bc 

0.1 a

 

Stylet Oil

38L

SF,F,1C,2C

10.2 a

7.5 c

Stylet Oil

19L

SF,F,1C,2C

12.4 ab

8.5 d

propiconazole

0.29L ai

SF,F,1C,2C

31.7 d

0 a

triflumazole

0.10kg ai

SF,F,1C,2C

18.5 cd

0 a

untreated

81.5 e

0 a


1 Sprays were applied according to phenology from about 2 weeks after full bloom until 1 week prior to harvest. SF= shuck fall (April 23); F= fruit fly spray (May 7); 1C=first cover spray (May 21); 2C = second cover spray (June 7)

2 Means followed by the same letters are not significantly different according to Fisher’s Protected LSD test at P < 0.05.

3 Average percentage of damaged cherries in 100 fruits from each of four replications. Fruit with obvious pitting and/or lack of luster were considered damaged.


Table 3. Effect of sequential applications of JMS Stylet Oil, DMI fungicides, strobilurin fungicides, and sulfur on severity of powdery mildew of sweet cherry at Orondo, WA 1999.

Treatment1

Rate/ha

Diseased

leaf area2, %

untreated

49.2 a2

Stylet Oil
microthiol

76L
1.14kg ai

31.0 b
 

Stylet Oil
calcium polysulfide

76 L
0.5% 19L

9.5 c 
 

Stylet Oil
triflumizole
trifloxystrobin

76 L
0.04 kg ai
0.11 kg ai

6.4 c 

 

Stylet Oil
trifloxystrobin

76 L v:v
0.11 kg ai

5.0 c

Stylet Oil
triflumizole

76 L v:v
0.04 kg ai

8.3 c

trifloxystrobin3
triflumizole

0.11 kg ai
0.04 kg ai

7.6 c


1 Stylet Oil was applied at the first visible evidence of powdery mildew. Subsequent applications were made at intervals dictated by the secondary infection component of the Gubler-Thomas grape powdery mildew model.

2 Means followed by the same letters are not significantly different according to Fisher’s Protected LSD test at P < 0.05.

3 Industry-standard calendar spray schedule used.

Disease severity ranged from 49.2% infected leaf area in the untreated control to 5.0% for the Stylet Oil-trifloxystrobin treatment (Table 3). Trifloxystrobin and triflumizole applied according to the "industry-standard" calendar method of scheduling fungicide applications resulted in 7.6% infected leaf area. The use of strobilurin and DMI fungicides after the initial oil spray significantly reduced disease severity compared to the untreated control and to treatments utilizing only flowable sulfur after the initial oil treatment. The efficacy of calcium polysulfide was similar to that obtained with DMI or strobilurin sprays following the initial oil spray. There was no oil-induced damage to fruit or foliage in any treatments.


Use of Spray Oils in Cherry Nurseries in Eastern Washington

Sprays for the control of powdery mildew were applied to first-leaf cherry (cv. Bing) trees in a nursery located about 5 km west of Quincy, WA (Table 4). Sprays were applied to drip with a backpack sprayer operating at about 17.4 kg/cm2. Treatments were applied at 2-wk intervals from late May and until late July. Treatments consisted of 10 trees in each of four replications arranged in a randomized complete block design. Foliar mildew severity was assessed on 1 August and 1 September by randomly selecting five terminal shoots on each of five trees (located in the middle of the replication) and counting the number of mildew colonies on 10 leaves beginning at the first fully expanded leaf from the shoot apex. Disease severity data were subjected to analysis of variance and means separated with Fisher's Protected LSD test (P < 0.05).

Disease severity in the untreated control was 6.0 and 17.3 colonies/leaf at the first and second evaluations, respectively (Table 4). Disease severity ranged from 3.0 in the Orchex 796 treatment to 7.1 colonies/leaf in the fenarimol treatment at the first evaluation and from 3.6 in the Stylet Oil treatment to 15.0 colonies/leaf in the fenarimol treatment at the second evaluation.


Table 4. Effect of spray oils, soap, and DMI fungicides on the severity of sweet cherry powdery mildew in Eastern Washington nurseries, 1997.

Treatment1

Rate/ha

Leaf disease severity2

 

1 August

1 September

Stylet Oil

76L

3.3 a3

3.6 a

Orchex 796

76L

3.0 a

3.7 a

myclobutanil

140g ai

3.7 a

10.7 b

fenarimol

0.73L ai

7.1 c

15.0 c

calcium polysulfide

19L

3.2 a

9.8 b

M-Pede

76L

5.0 ab

12.7 bc

untreated

6.0 bc

17.3 d


1
Sprays were applied at 14-day intervals beginning at the first signs of powdery mildew.

2 Number of powdery mildew colonies per leaf.

3 Means followed by the same letters are not significantly different according to Fisher’s Protected LSD test at P < 0.05.


Conclusions

Our results agree with those of other studies on powdery mildews (3-11, 14-17) indicating spray oils are effective against this important group of plant pathogens. On cherry, spray oils applied alone early in the growing season and as companion products for fungicides that have a high potential for selecting resistant populations of pathogens are excellent tools for the management of cherry powdery mildew in both orchards and nurseries. In all instances, oils used alone provided control equal or superior to that obtained using DMI or strobilurin fungicides. Fruit and foliar damage can occur when sequential applications of spray oils are made from bud burst through harvest. However, limiting oil application to no later than the pit-hardening stage minimizes damage to foliage and fruit and makes the products commercially acceptable to growers and field support personnel.

Fungicide alternation programs consisting of one or two early spray oil applications followed by strobilurin and DMI treatments at 14-day intervals provided disease control superior to that obtained with DMI fungicides used alone. This approach also resulted in a reduction of fruit damage from 7.5 to 8.5% in the bud burst to harvest spray oil program to less than 0.3% in the spray oil, DMI, strobilurin program. Using this approach, the number of applications of DMI or strobilurin fungicides was limited to no more than two per season, which conforms to the fungicide resistance management guidelines developed and supported by the Fungicide Resistance Action Committee. The Washington sweet cherry industry is rapidly adopting this approach to powdery mildew management.

The use of spray oils in weather-based spray programs also provides a means of limiting the number of fungicide applications and managing resistance to DMI and strobilurin fungicides. Early oil application apparently suppresses growth and/or sporulation of the initial mildew colonies (15) and thereby reduces secondary inoculum production. There are additional benefits to this approach. First, by making the initial spray oil application at the first sign of powdery mildew, the grower knows that the application is necessary because the disease is present, as opposed to a prediction of primary infection. Second, spray intervals can be adjusted according to potential risk of secondary infection, which could reduce the number of fungicide applications and provide an additional economic benefit to the grower. Finally, reducing the number of fungicide applications will provide an environmental benefit through reduced pesticide use. Potential economic risks associated with this program include the high-cost of labor involved with regular orchard surveys to search for early powdery mildew signs and the cost of training field support personnel to properly diagnose the disease at its earliest stages, which is critical to making the initial treatment and preventing the production and spread of secondary inoculum. This management approach may also require grower investment in on-site weather monitoring equipment.

Oils provide an excellent means of disease control in the nursery and offer an alternative to DMI and other fungicides that have a high potential for selecting resistant populations of pathogens. Including spray oils in powdery mildew management programs provides a cost-effective alternative that helps reduce the risk of fungicide-resistant strains of the pathogen from developing and spreading to other areas.

Acknowledgements

The authors appreciate the support of this project by the aforementioned chemical manufacturers, Adcon Telemetry, Oregon Cherry Commission, Washington State University Agricultural Research Center, Washington State Commission on Pesticide Registration, Washington State Tree Fruit Research Commission, and Van Well Nursery.

Literature Cited

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