March 2001

Disease Control By A.R. Chase

Downy mildews have gained a strong foothold in the horticultural industry in the United States and Canada over the past few years. During the winter and spring of 2000, some new crops were hit by downy mildew, including red salvia (S. splendens) and pincushion flower (scabiosa), as well as crops that have been adversely affected for more than five years (alyssum, snapdragon, stock, pansy and rose). Little research has been conducted on recognition and control of this difficult group of diseases. The following research was partially supported by a grant from the Washington State Department of Agriculture for 1999 to 2000 as well as many fungicide producers.

Diagnosis

Recognizing the disease is the most critical part of any control program. This task becomes more difficult when new diseases appear. Downy mildew fungi cause a range of symptoms on different hosts, making the disease difficult to diagnose. We have developed a color chart to aid in recognition of the range of symptoms that downy mildew fungi can cause. (The chart can be obtained at cost from Chase Research Gardens Inc.)

Characteristics of downy mildew disease

A quick review of the literature on downy mildew disease reveals some serious gaps in the information that is available (Table 1). The effects of temperature and humidity on downy mildew as well as the method of transmission (seed-borne or soil-borne) and the time for a complete infection cycle are published, where known. However, the only ornamental plants that have received any research attention are rose and snapdragon. Even on such important crops as these, very little is known of the basic biology, spread and control of the downy mildew pathogens.

Cultivar response

A series of cultivar tests was run on snapdragons and alyssum. The results presented here are the summary of two tests on alyssum (Table 2) and four tests on snapdragons (Table 3). Alyssum cultivars were very susceptible to downy mildew in our trials. There were no cultivars with low disease susceptibility, and the majority showed medium to high susceptibility. This may be a function of the plant structure as well as basic disease sensitivity. The tight configuration of alyssum leaves creates an ideal environment for downy mildew sporulation and spread by increasing relative humidity around leaves. Downy mildew on alyssum is only manageable through the use of preventive fungicide applications.

The snapdragon cultivars tested were a mixture of garden and cut flower types. Trends in the snapdragon data indicate that certain flower colors had lower disease susceptibility than others. The white cultivars have lower disease ratings than any other color. Orange and yellow generally are medium-low or low in disease response. Red and purple cultivars are variable in response but generally ranked in the higher disease categories.

It might be possible to choose snapdragons with lower susceptibility to downy mildew; however, the relationship of color to susceptibility would make a complete product mix difficult to choose based on downy mildew susceptibility alone. In addition, no single line of snapdragons was consistently “resistant” to this disease. Those in the ‘Tahiti’ series were found in all disease severity groupings. The ‘La Bella’ line was mainly found in lower levels of disease severity, but the ‘Rocket’ series was also found in all severity groupings.

Fungicide Evaluations

A large number of fungicide trials were completed in 1999 and 2000. The Washington State Agricultural Department funded approximately 50 percent of the trials reported in this article. Results from seven snapdragon tests, four alyssum tests and two stock (Matthiola incanae) tests are presented in Table 4. Finally, Table 5 presents the products in an overall efficacy ranking.

The relationship between chemical class and efficacy in downy mildew control on ornamentals is obvious. Some of the most effective classes only have a single product for use on ornamentals. The best overall product in our tests was the organic phosphate fungicide fosetyl aluminum (Aliette). Aliette is fully systemic, allowing movement upwards and downwards and optimizing control of downy mildew. This product has been around for nearly 20 years. It is not always effective for downy mildew control when used after severe infection or at high rates. Our tests indicate optimal control when used at either 1 or 2 lbs/100 gallons.

Dimethomorph is a cinnamic acid derivative that has been combined with mancozeb into Stature MZ. This product should be available in the next three months and represents new chemistry for the control of downy mildews on ornamentals. The combination with mancozeb should aid in management and development of disease resistance. It has worked extremely well in two years of testing at Chase Research Gardens.

Finally, the strobilurin group of fungicides has several products with very good to excellent control of downy mildew on ornamentals. Three years of testing with Heritage (azoxystrobin) indicate slightly better control of downy mildew than with Compass (trifloxystrobin). This is attributable to the differences in the systemic nature of the two products. Heritage is upwardly systemic Ð moving from roots to stems and leaves or lower leaves to upper leaves. In contrast, Compass is locally systemic (mesostemic). Because downy mildew infections start within the leaf tissues and may be quite advanced without obvious outward signs or sporulation, Heritage can penetrate and give slightly better control than Compass.

Many other products have been tested, including biologicals, triazoles, carbamates, coppers and other miscellaneous categories. Generally speaking, carbamates such as Protect T/O and Dithane Rainshield provided good to very good control in our trials. Copper compounds can provide good control, but are not good first choices for downy mildews. One thing we found out with coppers on alyssum is that they are not safe under most conditions. Phytotoxic symptoms include slight to severe burning of leaf margins. I do not recommend use of copper fungicides for alyssum.

Other important questions were answered, such as which fungicides work best for downy mildew control on ornamentals. Our tests showed that additions of either non-ionic or silicon-based wetting agents did not improve performance of the products tested. Curative abilities of the fungicides tested are minimal. Generally, the product must be applied prior to infection.

In summary, downy mildew diseases are becoming more severe with previously undescribed diseases reported each year. The ability to recognize these new diseases prior to severe infestations is limited and thus, our ability to control losses is limited. Once a disease is recognized on a crop, control must be based on prevention. Rescue treatments even with products such as Heritage, Aliette and Stature MZ that are 100 percent effective when used preventatively are occasionally successful, but more often than not are a serious disappointment.