Protect your plants from Powdery Mildew

Figure 1: PM on Tomato

 Powdery mildew is a fungal disease that affects a wide variety of plants. The affected plants have a dusted-with-flour appearance with white to gray powdery spots on the surface of leaves and fruits (Fig 1). It causes cosmetic damage, and defoliation and decreases the quantity and quality of fruit (Fig. 2).

Disease and Development 
There are many different species of (Powdery Mildew) fungi. Most species only attack specific plants in the same or closely related plant families.

Figure 3: PM on roses

For example, powdery mildew on roses does not spread to grapes.

Powdery Mildew fungi overwinter in buds, twigs, bark, and plant debris. Their spores are carried by wind to new hosts. Low relative humidity during the day favors spore dispersal.

Fig 2: PM on grapes

The optimum conditions for spores to germinate are temperatures between 68 to 77°F and high relative humidity (> 70%). Powdery Mildew spores can germinate in the absence of water and can infect in arid conditions (40% humidity). Water on plant surfaces can kill or inhibit germination. Spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight. Shady conditions are favorable for PM development.

Management
The disease triangle is a useful tool to understand and manage diseases like Powdery Mildew (Fig. 3). The severity of Powdery Mildew infections is a function of the presence of the pathogen, the host susceptibility, and the environment. Powdery Mildew can be managed by good cultural practices.   

Fig 3: The Disease Triangle

The best control is to plant only resistant species or cultivars. Avoid excess fertilizer since plants with abundant succulent tissues are more susceptible to infection. The fungi can be eliminated by removing and burning all infected plant tissues and debris. Tools must be disinfected after use.

Another good control method is making environmental conditions less favorable for the disease. Avoid dense canopies by proper plant spacing and pruning to provide good air circulation, reduce humidity and increase sunlight penetration. Plants should not be planted in poorly drained soils or shady damp areas.

Overhead watering of the leaves during the day and when humidity is low can reduce infection. Water washes spores off the plant and inhibits its germination. This control method is not recommended for vegetables since it may contribute to other pest problems.

Powdery Mildew seldom needs chemical control in residential plantings, but it may be warranted in commercial plantings of susceptible crops like grapes and cucurbits. Chemicals are most effective when combined with cultural control. They should be applied before the disease symptoms appear. Further spread may be stopped or slowed with chemical treatment. Materials like horticultural oil or neem oils have a low environmental impact. Apply every 7 to 14 days throughout the season. Do not apply to drought-stress plants when the air temperature is above 85° nor within 2 weeks of a sulfur spray. Wettable sulfur is very effective in preventing infection. Like oil application, it should not be applied when the air temperature is over 90° nor within 2 weeks of an oil application.

The Best Time to Prune Grapevines in Southwest Idaho

Pruning grapevines is an essential practice for ensuring healthy growth and a productive harvest. However, in regions like Southwest Idaho, timing is everything. Pruning too early in the season can expose vines to late winter freezes, potentially damaging buds and reducing fruit production. To maximize your grape yield and maintain vine health, it’s crucial to understand the best time and method for pruning in our local climate.

Why Timing Matters

Grapevines go dormant in the winter, making it a prime time for pruning. However, Southwest Idaho's climate presents unique challenges. While some gardeners may be tempted to start pruning in January or February, doing so can leave the vines vulnerable to unexpected cold snaps. If a sudden freeze occurs after pruning, exposed cuts and early bud development may be damaged, leading to reduced fruit production in the coming season.

The best time to prune grapes in Southwest Idaho is typically in late winter to early spring, around March, when the risk of deep freezes has diminished. By waiting until temperatures stabilize, you help protect your vines from cold damage while still pruning before the growing season begins.

How to Prune Your Grapes

1. Assess Your Vine – Begin by evaluating the structure of your vine. Identify dead, diseased, or weak wood that should be removed.

2. Choose a Pruning Method – The two most common methods are cane pruning and spur pruning.

   

   • Cane Pruning: Best for varieties that produce fruit on one-year-old wood. Select one or two healthy canes from last year’s growth and remove the rest.

   • Spur Pruning: Suitable for varieties that bear fruit on short spurs. Keep several short, two- to three-node spurs along the main cordon.

3. Make Clean Cuts – Always use sharp pruning shears to make precise cuts. Cut just above a bud to encourage healthy new growth.

4. Train the Vine – As you prune, shape the vine to ensure proper airflow and sun exposure, which help prevent disease and improve fruit quality.

5. Monitor Weather Conditions – Keep an eye on the forecast. If a hard freeze is expected after pruning, consider providing some protection, such as mulching around the base of the plant.

Growing Grapes in Southwest Idaho

In addition to proper pruning techniques, successful grape growing in Southwest Idaho requires careful selection of grape varieties suited to the climate, well-draining soil, and adequate sunlight exposure. Proper trellising, irrigation, and pest management also play key roles in maintaining healthy vines and producing quality fruit. For further guidance on growing grapes in this region, including best practices beyond pruning, check out extension publications from the University of Idaho, Utah State University, and Oregon State University.

Final Thoughts

Proper pruning is key to successful grape growing in Southwest Idaho. By waiting until late winter or early spring to prune, you help safeguard your vines against freeze damage and set the stage for a bountiful harvest. Whether you’re growing table grapes or wine grapes, careful pruning will encourage healthy growth and improve fruit quality for seasons to come.

Powdery Mildew

Figure 1: PM on Tomato

Powdery mildew is a fungal disease that affects a wide variety of plants. The affected plants have a dusted-with-flour appearance with white to gray powdery spots on the surface of leaves and fruits (Fig 1). It causes cosmetic damage, and defoliation and decreases the quantity and quality of fruit (Fig. 2).

Disease and Development
There are many different species of (Powdery Mildew) fungi. Most species only attack specific plants in the same or closely related plant families. For example, powdery mildew on roses does not spread to grapes.

Powdery Mildew fungi overwinter in buds, twigs, bark, and plant debris. Their spores are carried by wind to new hosts. Low relative humidity during the day favors spore dispersal.

Fig 2: PM on grapes

The optimum conditions for spores to germinate are temperatures between 68 to 77°F and high relative humidity (> 70%). Powdery Mildew spores can germinate in the absence of water and can infect in arid conditions (40% humidity). Water on plant surfaces can kill or inhibit germination. Spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight. Shady conditions are favorable for PM development.

Management
The disease triangle is a useful tool to understand and manage diseases like Powdery Mildew (Fig. 3). The severity of Powdery Mildew infections is a function of the presence of the pathogen, the host susceptibility, and the environment. Powdery Mildew can be managed by good cultural practices.   

Fig 3: The Disease Triangle

The best control is to plant only resistant species or cultivars. Avoid excess fertilizer since plants with abundant succulent tissues are more susceptible to infection. The fungi can be eliminated by removing and burning all infected plant tissues and debris. Tools must be disinfected after use.

Another good control method is making environmental conditions less favorable for the disease. Avoid dense canopies by proper plant spacing and pruning to provide good air circulation, reduce humidity and increase sunlight penetration. Plants should not be planted in poorly drained soils or shady damp areas.

Overhead watering of the leaves during the day and when humidity is low can reduce infection. Water washes spores off the plant and inhibits its germination. This control method is not recommended for vegetables since it may contribute to other pest problems.

Powdery Mildew seldom needs chemical control in residential plantings, but it may be warranted in commercial plantings of susceptible crops like grapes and cucurbits. Chemicals are most effective when combined with cultural control. They should be applied before the disease symptoms appear. Further spread may be stopped or slowed with chemical treatment. Materials like horticultural oil or neem oils have a low environmental impact. Apply every 7 to 14 days throughout the season. Do not apply to drought-stress plants when the air temperature is above 85° nor within 2 weeks of a sulfur spray. Wettable sulfur is very effective in preventing infection. Like oil application, it should not be applied when the air temperature is over 90° nor within 2 weeks of an oil application.