§   Downy mildew on Lamium
§   How to visualize air flow
§  Powdery mildew on verbena is stubborn but can be managed
§   Downy mildew showing up on impatiens

Downy mildew on Lamium
Jan Byrne, MSU Diagnostic Services

Diagnostic Services received a Lamium sample (Lamium maculatum ‘White Nancy’) with angular leaf spots, particularly on the lower leaves. These plants were infected with Peronospora lamii, the fungal pathogen that causes downy mildew. The leaf spots were often delineated by the leaf veination.  The pathogen was readily sporulating on the underside of the foliage, covering the undersides of the leaves with gray fuzzy mold.  I have also seen this problem on the cultivar Beacon Silver, in that case the lesions were purple in color.

Downy mildew is favored by cool, wet conditions.  Spores are produced under conditions of high relative humidity and are subsequently released in large numbers when relative humidity drops (particularly in the morning hours).  Spores are carried through the air, spreading the disease under favorable environmental conditions.

Several fungi cause downy mildew – the good news is that they are all fairly host specific.  Peronospora lamii, infects lamium and salvia, and possibly a few other plants in the mint family.  Plants such as roses, impatiens, pansies and snapdragons can also have downy mildew disease problems, but each of these hosts has its own downy mildew fungus.

Cases of confirmed downy mildew will require fungicide applications for disease control. Good fungicide coverage, including the undersides of the leaves, is needed to control this pathogen.  The fungicide Stature DM 50WP can be used in conjunction with protectant fungicides to achieve disease control. Several applications will likely be needed.  Please note the additional instructions in the resistance management section of the label.  Growers with severe disease problems in a limited number of plants should consider destroying the affected plants.  

How to visualize air flow
Dean Krauskopf
Integrated Crop Management Agent, Southeast Michigan

Reducing humidity around leaves and flowers can be critical to controlling Botrytis and other diseases under cool, cloudy conditions. Here’s a quick way to check air movement in your greenhouses from Dr. Peter Ling, Greenhouse Engineer, The Ohio State University, OARDC, Wooster, OH.

Good air circulation is important for plant production in greenhouses. The air movement helps to create a more uniform environment for plant growth. It also helps to remove high humidity from the plant canopy, which helps reduce diseases.

How much airflow is enough but not too much? A general rule of thumb is that airflow should be strong enough to move plant leaves slightly. The method works well with plants that have long branches, but it is more challenging for younger, bedding plants. An alternative is to use a ribbon or tissue paper to see air flow.

Hold still a piece of ribbon or tissue paper (two to three inches long) over the plant canopy. Slight movement of the piece is an indicator of sufficient airflow. One can use this simple indicator to adjust direction, location and speed of the horizontal airflow (HAF) fans to achieve the best air circulation pattern. Remember, air moves not only horizontally but also vertically. Having plants on benches and having breathing holes on trays will help vertical air flow.

Powdery mildew on verbena is stubborn but can be managed

Powdery mildew is a common and difficult problem on Verbena if not managed effectively. This disease is caused by a fungus (Erysiphe cichoracearum) and can infect many different annual and perennial flowers. Powdery mildew can occur on all aboveground plant parts and results in white growth on the plant’s surface. When the fungus reproduces, the abundant conidia (spores) give a white, powdery or fluffy appearance. Severe infection can cause yellowing and withering of leaves and restrict plant growth. Powdery mildew can infect plants even when the relative humidity is low, but epidemics are prompted when relative humidity is high. Sometimes the powdery mildew progresses unnoticed until many plants are infected. We were interested in determining which products are most effective when used preventively (prior to disease) or once powdery mildew is established. It was also of interest to determine how long the products remain effective and provide protection against disease following application.

In the first trial, a fungicide treatment was applied before any powdery mildew was evident. Two additional applications were made at 14-day intervals. After the third spray, the plants were evaluated and the severity of powdery mildew determined. Two additional evaluations were made 25 and 45 days after the last fungicide spray to see how long the treatments would provide protection. The amount of foliage diseased was estimated using a rating system with 1=no disease, 2=trace amount to 10%, 3=10 to 20%, 4=20 to 30%, 5=30 to 40%, 6=40 to 50%, 7=50 to 60%, 8=60 to 70%, 9=70 to 80%, and 10=80 to 100%.

At the end of the first trial when sprays were initiated before powdery mildew was observed, plants that were not treated became severely diseased and received a rating of 10 (at least 80% of the foliage was diseased). While most treatments offered some benefit, fungicides such as Terraguard (4 or 8 oz. rate), Compass 50WG, Strike 50WDG, and Systhane 40WP offered exceptional control (rating of 1; no disease). Even 25 days after the last spray, powdery mildew was limited to a rating of less than 2 (trace amount of disease). The protection provided by all of the products included in this trial began to breakdown 45 days after their application and powdery mildew began to develop. Products such as Spectro 90WDG and Camelot 58% EC might have benefited from a closer application interval of seven days compared with the 14-day interval that we used in this study.

In the second trial, the first fungicide treatment was delayed until powdery mildew developed on the foliage. A second fungicide application was made 14 days later. Disease evaluations were made as described for the first trial and plants that were not treated became severely diseased (at least 80 percent of the foliage had powdery mildew). Once it was established, powdery mildew was difficult to eradicate in this trial even with two fungicide sprays. However, several fungicides kept powdery mildew limited to a rating of less than two (trace amount to 10 percent disease) for 25 days after the last application and included Terraguard (4 or 8 oz. rate), Compass 50WG, Strike 50WDG, and Systhane 40WP. Powdery mildew began to develop on all treatments 45 days after the last fungicide application. It is possible that a third or fourth spray might have eradicated the powdery mildew.

Although powdery mildew can offer a real challenge to growers of susceptible crops such as Verbena, there are many new tools that offer excellent control. According to our results, Compass 50WG, Terraguard 50W, Strike 50WDG, and Systhane 40WP are highly effective in controlling powdery mildew even when used at two to three week intervals. While all of these fungicides list powdery mildew on their label, Verbena is specifically listed only on the Compass 50WG label. Powdery mildew is well known for its ability to adapt and develop resistance to fungicides and it is important to rotate among fungicides with different modes of action. Using as few sprays as possible also helps to delay the development of resistance. Our study also suggests that powdery mildew is best controlled when fungicides are begun prior to the appearance fo the disease. Since Verbena is so susceptible to powdery mildew, it would likely be of benefit to begin a spray program prior to disease development and apply fungicides (alternate products) effective against powdery mildew at two to three weeks’ intervals to maintain a healthy crop.

Acknowledgment: This research was funded in part by the American Floral Endowment.

Dean Krauskopf
Southeast Michigan Integrated Crop Management Agent

If possible, destroy infested plants. Place them into plastic trash bags, seal the bags before taking them out of the greenhouse to prevent the spread of spores and either burn them or place them in a landfill. Dr. Mary Hausbeck in the February 25, 2004 issue of the Greenhouse Alert recommended the following weekly spray schedule:

Growers who have no cutting propagated impatiens but did buy in impatiens plugs should consider applying preventive sprays since the plugs may have been exposed to infected cutting propagated impatiens. The plugs may have appeared healthy but could develop the disease if action isn’t taken.