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.

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.

Salad Tomatoes, Small but Mighty in Flavor & Production

Good things come in small packages. This platitude rings true for saladette tomatoes, commonly known as salad tomatoes. Their small, ping pong ball form and robust flavors deserve a special place in your garden.

Often overlooked when compared to big beefsteaks or cherry tomatoes, salad tomatoes generally grow larger than cherry tomatoes but smaller than medium-to-large paste and slicer varieties. Many varieties of salad tomatoes possess a relative tolerance to cold, so you can plant them outside when cooler temperatures prevail and other less hardy varieties must still bask in heated greenhouses. No variety of tomato, however, is frost tolerant.

Famed tomato breeder Ben Quisenberry created the Large Red Cherry, that he claims can be canned whole - seeds and all. His unique variety differs from the smaller, more commonly available tomato "Red Cherry, Large.” Lesser-known salad tomato varieties include: Garden Peach, Tigerella and Black Vernissage. As indeterminates, all three varieties require staking and pruning. Garden Peach, a "long keeper" tomato, stores well when picked green just before frost arrives in autumn. Excluding their green stems, the ripened fruit look like small, slightly fuzzy peaches. Tigerella, an English heirloom tomato, wears beautiful orange stripes over a red skin and sets fruit early, often three weeks before than other salad varieties. Immature fruits bear light and dark green stripes. Tigerella demonstrates a degree of cold tolerance and disease resistance, too. Black Vernissage tomatoes sport a deep mahogany skin with subtle green stripes. Excellent for drying, eating fresh and making tomato sauce, this variety remains productive throughout the entire season.

Another benefit of selecting a salad type tomato for your garden is resistance to the dreaded Beet Curly Top virus, a known scourge to tomato plants in the Treasure Valley. The Payette and Owyhee varieties of tomato developed in the 1960s by University of Idaho show resistance to the virus. They grow well in our region, too.

Salad tomatoes have earned the right to be known as the garden workhorse. They produce prolifically and ripen fruit early compared to other non-cherry varieties, making them ideal for higher latitude and altitude gardens. During years of difficult weather conditions, they can be an insurance policy to ensure a tomato harvest. Salad tomatoes also fill multiple niches in the kitchen, performing equally well for canning, making pasta sauce, or used fresh in salsa. 

Branch out, experiment, and plant new tomato varieties this season. You can’t go wrong growing salad tomatoes in your garden.