November 2000

Combating Powdery Mildew: A Progress Report for the Year 2000 By A.R. Chase

There have been some interesting new developments in the fight to control powdery mildew on greenhouse ornamentals since my last article on the subject (GPN February 1999). The number of plants suffering from powdery mildew continues to increase each year as new crops are added to our product mixes. This is especially true for powdery mildew diseases on specialty cut flowers and perennials. Table 1 contains a listing of some powdery mildew diseases on several groups of ornamentals.

Some of the newest work has been performed on woody ornamentals including poinsettia, gerber daisy, hydrangea, dogwood, and many types of roses. Over the past three years more than a dozen trials have been reported on these crops, with the majority of trials performed on roses. The figure and tables in this article summarize these results as well as those for tests performed by Chase Research Gardens, Inc.

GERBER DAISY AND POINSETTIA

Margery Daughtrey (Long Island Horticultural Research Laboratory) performed a trial in 1999 evaluating Heritage (1, 2 or 4-oz/100 gal). This strobilurin fungicide failed to control development of powdery mildew on leaves but gave excellent control of disease on bracts. Triact 70EC (0.5 percent on a seven-day interval), Serenade EC (3 percent on a seven-day interval) and Terraguard 50WP (8 oz on a 14-day interval) each provided excellent powdery mildew control on both leaves and bracts. Triact did cause some phytotoxicity but the other products were safe in this trial.

We have extensively tested the control of powdery mildew on gerber daisy over the past four years. Table 2 summarizes tests for 1996 through 1999. In late 1999 and early 2000, several other trials were completed. Cinnamite was used preventatively at 64 and 85 oz/100 gal on seven, 14 and 21-day intervals. It provided excellent control at 64-oz on a 14-day interval and lasted for 21 days when used at 85-oz/100 gal. BAS114UBF (a formulation of Milsana, a botanical extract) at 0.5 or 1 percent, or Strike 25W at 2 oz failed to give control. In the same trial, Triact 70EC provided excellent control when used at 0.5 or 1 percent. In the most recent trial, Cinnamite (64-oz/100 gal) provided good control, while Pipron and Rubigan, each at 4 oz/100 gal, gave excellent control. Rubigan caused stunting and leaf distortion typical of other sterol inhibitors on gerber daisy.

HYDRANGEA AND DOGWOOD

Two hydrangea trials were performed for the control of powdery mildew caused by Erysiphe polygoni. The first was performed on plants without active powdery mildew (preventative); control was excellent with Eagle (same active ingredient as Systhane) and Heritage, and very good to excellent with 3336. Heritage applied on a 21-day interval at 4-oz/100 gal completely prevented development of powdery mildew in this trial.

The other trial was performed on hydrangea with active and severe powdery mildew. While I wouldn’t advise waiting until severe disease occurs to apply a fungicide, this trial showed that many fungicides can act in a curative fashion (see Figure 1 on pg. 15).

The best control in the trial was achieved with Systhane 40W applied at 3-oz/100 gal every 14 days. Phyton 27, Strike and Triact 70EC also provided very good to excellent control. TopShield did not provide any curative action against powdery mildew caused by E. polygoni.

Five trials were reported for powdery mildew (Phyllactinia and Microsphaera) control on dogwood. In these tests, most fungicides showed very good to excellent control when applied on a 14-day interval. Banner Maxx (8-oz), Bayleton (4-oz), Eagle (6 to 8-oz), and Lynx (2.22-oz) all belong to the sterol inhibitor fungicide group. This group is very active against powdery mildew diseases.

Domain and 3336 (both benzimidazoles) also gave very good to excellent control.

Likewise, Heritage (a strobilurin fungicide) gave very good to excellent control when used at rates of 4- or 8 oz on a 7-, 14- or 21-day interval.

ROSE and MINIATURE ROSE

Nine trials were conducted for powdery mildew control on both garden and miniature roses. Pipron gave excellent control when used on a seven-day interval. Overall, the three bicarbonate products (eKsPunge, FirstStep and Kaligreen) provided very good to excellent control against powdery mildew on roses when used on a seven-day interval.

The sterol inhibitors (Systhane, Terraguard and Strike) also gave very good to excellent control Ð usually on a 14-day interval.

Copper products provided varying results: Phyton 27 gave excellent control, Camelot showed very good control and Junction provided good control.

The benzimidazole product (3336) gave excellent control when applied weekly.

All representatives of the strobilurin group (Compass, Cygnus and Heritage) gave some degree of control (even very good control) when used on a 14-day interval at rates of approximately 2-oz/100 gal. BAS114UBF (a formulation of Milsana) gave very good control when used at 0.5 percent.

Triact (Neem extract) showed excellent control when used at 1 percent on a 14-day interval.

Cinnamite (botanical extract) provided very good control and Serenade (bacterial biocontrol agent) gave excellent control when used on a seven-day interval. The fungal biocontrol agent TopShield also gave good control.

CONCLUSIONS

One of the most interesting things I discovered during trials I conducted on powdery mildew control was that powdery mildew fungicides can be classified into at least eight distinct chemical groups (Table 4). This diversity allows most growers to rotate between the groups as a means of delaying or stopping the development of resistant powdery mildew fungi. Some of the newest products to enter the market are in the strobilurin group (Compass, Cygnus and Heritage). These product labels specifically describe resistance management strategies required to ensure that these products retain their efficacy over the long haul. What this means is that growers really have no excuse for relying on a single chemical group to control powdery mildew diseases on the majority of ornamental crops.

Indeed, the breadth of chemical groups allows growers to use “soft” chemicals such as biologicals, botanical extracts and bicarbonates. The designation “soft” refers primarily to the environmental “friendliness” of this group. Although considered “soft,” these products should still be tested on your crops for safety before broad-scale use in the greenhouse or nursery. When chosen and applied in an informed manner, each of these products have a place in the ornamental industry.

Author’s Note: The previous report was not meant as an exhaustive review of either published reports on powdery mildew control or a listing of all products used for powdery mildew control on ornamentals. Apologies are extended to companies with products not discussed. This article in no way constitutes a recommendation of one product over another and is solely meant for educational purposes. Remember: the label is the law!

KEY POINTS TO COMBATING POWDERY MILDEW

1. Powdery mildew colonies appear on surfaces of leaves, petioles, stems and flowers.

2. Powdery mildew fungi are host specific Ð the fungi on rose cannot attack gerber daisies or vice versa.

3. Powdery mildew is most common outdoors during the spring and fall.

4. Powdery mildew is inhibited by frequent rainfall (or overhead irrigation) and cold or hot conditions.

5. Powdery mildew spores move by air currents (wind or fans).

6. Rotate between chemical classes to stop resistance development.

7. Scout your crops to time sprays before an epidemic starts.