Soil and water

Master gardeners advise on soil moisture and zucchini mildew

Question: I know we are experiencing a severe drought this year and I want to save water, but I also don’t want my plants to die off. How do I know if I am giving them enough water?

Answer: A great question and a question everyone is concerned about right now! The answer depends on the types of soil and plants you have in your garden and how much sunlight they get each day.

Your soil type can be sand, silt, clay, or silt or a combination of these types. Water moves quickly in sandy soil but moves slowly in clay soil. Clay retains a significant amount of moisture, while sandy soil dries out quickly. In general, water sandy soils quickly, for short periods. Water clay soils slowly, until the soil no longer absorbs water and water begins to drain.

Try this experiment in your garden: water your soil as you normally would, then after an hour dig 6-8 inches to see how far the water has seeped. If the water in your garden reaches below the root zone of your plants, you are wasting water.

Overhead sprinklers spray water into the air, where it evaporates, or onto sidewalks, driveways and tree trunks. The most efficient way to water your plants is through drip irrigation.

After July, although it is still warm, the days get shorter and the plants need less water. If you watered for 10 minutes in July, you may be able to reduce to 8 minutes in August and 6 minutes in September.

Unlike young trees which require frequent watering to thrive, mature trees prefer less frequent but deeper watering in the summer. Native trees and shrubs are drought tolerant. Depending on the precipitation in winter, they do not require water or water once or twice during the summer months. Native oaks should not be watered in summer.

Check with your water supply agency for current water use restrictions.

Useful links:

How to program your sprinkler timer:

Santa Rosa Water Smart:

Watering recommendations:

Question: Everyone says zucchini are easy to grow, but the leaves of my zucchini are coated with a white powdery substance and the young squash rots from the flowering tip. What should I do?

Answer: It seems that your zucchini is suffering from a combination of powdery mildew and blossom end rot. These are two very different diseases that you can treat by improving the growing conditions of your plants.

Powdery mildew is a fungal pathogen that, if given the opportunity, will colonize a variety of vegetables, such as artichokes, beans, beets, carrots, cucumbers, eggplants, lettuce, melons, peppers, radishes, squash, tomatoes and more. It thrives in our Mediterranean climate because it does not require humid conditions and favors hot summer days.

Powdery mildew on cucurbits (cucumbers, melons, squash, and pumpkins) first appears as pale yellow spots on the stems and leaves. The spots enlarge as a fluffy white fungal mat (mycelium) that grows and produces powdery spores. Affected leaves appear dull and begin to fade, eventually turning brown and papery. Plant death often occurs at this stage.

The best way to fight powdery mildew is to avoid the conditions that cause it. Start by controlling weeds and removing garden debris that harbor powdery mildew spores. Grow your zucchini plants in full sun, with adequate spacing between plants to allow air circulation. Find and plant varieties of zucchini resistant to powdery mildew.

We do not recommend overhead watering as this may encourage other pathogens or plant pests. Organic fungicidal treatments such as sulfur can be used as a preventive measure or to treat infection but should be applied at the latest at the first sign of disease. Once the growth of late blight is extensive, it is more difficult to control with fungicides.

Blossom end rot is a disorder of cucurbits, tomatoes, eggplants and peppers. It first appears as light brown spots at the flower tip of immature fruit that expand to form a dark, leathery, sunken lesion. Fruits with blossom end rot are deficient in calcium. However, adding calcium, whether through soil application or as a foliar spray, does not help.

Blossom end rot is a physiological response to environmental stresses, such as drought, soil salinity, intense light, heat, and over-fertilization. You need to take these factors into account to resolve the issue.

To avoid blossom end rot, take steps to reduce stress on plants. In particular, avoid fluctuations in soil moisture. Between waterings, do not allow the soil to dry out completely or remain saturated. If you use nitrogen fertilizer, use it only according to the instructions on the package.

For more information on powdery mildew and blossom end rot, visit the University of California Integrated Pest Management Program website at for more information on powdery mildew and for blossom end rot.

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