About Powdery Mildew
Powdery mildew is a fungus that can affect the growth of numerous different plant types. This mildew has different species such as erysiphe cichoracearum and erysiphe lycopersici; certain species affect certain plant types. In order to grow, this fungus needs live plant tissue. When the right conditions are present, powdery mildew of the species sphaerotheca fuliginea has the ability to overwinter as resting spores. This species is the most common and mainly affects beans, peas, and cucurbits. Most other varieties require host plants year-round for survival. Powdery mildew tends to grow in fine tissue layers on the infected plants surface. This white fungus is mainly composed of spores, which is how it is distributed. The spores are made in chains, which are visible through a hand lens. The spores travel by wind, causing them to spread to new host plants in the area. Germination for all species of powdery mildew can occur without water; some species will actually be hindered from the presence of water if it remains there for a long period of time.
This fungus will first appear as powdery white patches that are fairly small in size on the surface of host plants. It mainly targets the plants leaves but will sometimes travel to the fruit, flowers, or shoots. These spots slowly expand to cover a significant portion of the plant’s stems and leaves. The exception to this is the variety leveillula taurica, which will form spots that are more yellow in color and will not appear as powdery as the other varieties. When a plant has powdery mildew, leaves can turn yellow and brown, sometimes leading to death. When these dead leaves fall off of the plant, it can expose fruit to sunburn. This disease can also create twisted and buckled leaves. The fungi will not often grow on the plants fruit, although plants with this disease can have a decreased yield and not overly flavorful fruit. They also tend not to produce for as long as uninfected plants.
- • Erysiphe cichoracearum
- o Affects: cucumbers, lettuce, squash, potatoes, melons
- • Erysiphe cruciferarum
- • Erysiphe lycopersici
- • Erysiphe pisi
- • Erysiphe heraclei
- o Affects: parsley, carrots, parsnips
- • Erysiphe polygoni
- • Leveillula taurica
- o Affects: peppers, tomatoes, artichoke, eggplant
- • Sphaerotheca fuliginea
- o Affects: cucurbits, beans, peas
Due to this fungi’s ability to thrive in dry environments and its preference to warm weather, it is one of the more common leaf diseases in areas with dry, hot summers such as California. Temperatures between 60 and 80 degrees Fahrenheit are the most advantageous for this disease. Shady areas are also highly beneficial for spore development. Growth will become challenging for powdery mildew if temperatures exceed 90 degrees Fahrenheit, especially when in direct sunlight.
Powdery Mildew Management and Control Methods
Management of this disease is easiest when you use preventative measures. There are certain varieties of vegetables that have effective resistance. You can also avoid the varieties that are more susceptible to powdery mildew. Possible host plants should be placed in an area with good sun exposure, decent air circulation, and should not be given an excessive amount of fertilizer. If an overhead sprinkler is used, this can help manage the disease as spores are often washed away from the leaves. However, this method should be considered carefully because sometimes overhead sprinklers can make way to other plant pests.
When prevention methods are not enough to control powdery mildew, especially for plant varieties that are most susceptible such as cucurbits, fungicides are sometimes necessary for effective treatment. There are fungicides available for the use of both disease eradication and disease protection. Those used for treatment should be applied at the first sign of infection; the longer you leave the mildew, the harder it becomes to treat. Horticultural oils, sulfur, and biological fungicide options are all available for powdery mildew. The oils are best for treatment once an infection has been spotted, whereas sulfur and Serenade, the biological fungicide, are best for prevention. Possible spray oils include Sunspray Ultra-Fine, jojoba, neem, and Saf-T-Side. Note that oils should not be applied within 14 days after a sulfur spray as this can result in plant injury. Additionally, oils should not be applied in temperatures warmer than 90 degrees Fahrenheit. Keep in mind that sulfur sprays are only effective if applied before the disease breaks out, and that it tends to be more effective at prevention than Serenade, which is a bacterium that is meant to destroy the fungal pathogens of the disease. Always be sure to carefully read the label for cautions and proper application before use.