In this Summer 2025 issue of the UCNFA Newsletter, our Featured Articles focus of diseases of nursery crops, presenting the most recent diagnostics data from ornamental nurseries in California out of the Greenhouse and Nursery Pathology Lab at UC Davis. We will also discuss the impact and biology of powdery mildew in California nurseries, including practical guidelines for identification and management.

If you’ve ever seen a white, dusty coating on your ornamentals or nursery plants, chances are you’ve met powdery mildew, a common and frustrating disease affecting a wide range of nursery and floriculture crops.

Powdery mildew is caused by a group of fungi that are host-specific, meaning the one on your roses won’t necessarily infect your zinnias or begonias. Still, many ornamental crops are vulnerable, including roses, hydrangeas, snapdragons, phlox, chrysanthemums, begonias, kalanchoes, and many woody shrubs like crape myrtle, lilac, and dogwood.

Phytophthora species are recurrent pathogens infecting ornamental crops worldwide, with that, California is not the exception. From the disease diagnostics clinic from Del Castillo Lab, in 2024, Phytophthora root rot was the most prevalent disease diagnosed. Infected hosts include wax flower, gardenia, banksia, orchid rock rose, lavender, and boxwood, among other plants. From these plants, nine Phytophthora species were identified, with P. nicotianae, P. palmivora, and P. niederhauserii among the most common ones.

Recycled irrigation runoff water is a viable alternative for nursery and greenhouse production. However, it is likely to have plant pathogens present including: viruses, water molds, fungi, nematodes, and bacteria. To prevent plant pathogen spread, recycled irrigation water needs to be disinfected. Typical sanitation techniques include using chemicals (e.g. chlorine products or ozone), heat treatments, or ultraviolet light to kill pathogens.

The pH of soil and water and how it affects plant growth and chemical makeup of soils and media is somewhat detailed.  In the next five newsletters, we will present different aspects about pH.  Once complete, one should have a thorough understanding of pH and how it influences plant production.

Part I – What is pH?

Part II – How is nutrient availability affected by pH

Part III – How do fertilizers affect pH?

Part IV – How to adjust pH of soils

Part V – How to adjust pH of irrigation waters

Between the many production systems used in the nursery and greenhouse industry, a one-size-fits-all watering solution does not exist, even for a one-gallon pot. Still, there are common concerns when it comes to how water moves, where it lingers, and the impacts for plant health. While research and testing are critical for identifying specific pathogens like Phytophthora, Pythium, and Rhizoctonia, many outbreaks begin with simple environmental conditions that go unnoticed. For growers, recognizing water-related warning signs can be just as important as selecting a fungicide.

We are all feeling the pressure of summer heat, and so are our irrigation pipes! Fortunately, the pipes aren’t cracking (reminder to check your pressure regulators), and neither are California’s nursery growers. A recent hands-on irrigation training designed to help nursery growers improve water use efficiency and system performance was led by UC Cooperative Extension Advisors. The training provided both foundational knowledge and practical demonstrations tailored for container nursery systems.

University of California Cooperative Extension Advisors trained 107 nursery professionals during nine technical irrigation trainings at California nurseries in 2025. Trainings were conducted in either Spanish, English, or both languages, to accommodate the diverse workforce of the nursery industry. In the trainings, we covered the basics of plant physiology, soilless substrate science, and irrigation best management practices to improve water-use efficiency and plant health.

Phytophthora diseases are a growing concern to restoration and nursery plant production that have led to huge economic and ecological losses. Here in California, most people may know of Phytophthora (lit. “plant destroyer”) from the devastating outbreak of Sudden Oak Death, caused by the aerial Phytophthora ramorum, in the 1990s – 2000s in Northern California and Southern Oregon.