Powdery Mildew On Lupine Foliage: What to Do
If you’re familiar with white leaves on your lupine, you know powdery mildew is one of their most common afflictions. Gardening expert Katherine Rowe outlines how to prevent and treat the fungal problem to get the most ecological and ornamental value from the short-lived perennials.
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Lupines are ornamental perennials that play important roles in pollinator resources, soil improvement and stabilization, and erosion control. The beautiful spires of blue, purple, pinks, and yellows require few resources to thrive. There are some 300 Lupinus species, with most native to North America and the Western United States, in particular. The showy native species are rugged and adaptable, with numerous cultivars bred for exceptional blooms and landscape performance.
With all of their hearty wildflower merits, lupines are susceptible to common garden problems, powdery mildew being a top offender. Several pathogens of the fungus Erysiphe can crop up, including one specific to the genus, E. lupini, and one that impacts legumes, E. intermedia. If your lupine leaves are turning white, or to prevent them from doing so in the first place, a few countermeasures will help.
Setting Lupine Up For Success
Lupines grow beautifully in cool, mild climates. They’re short-lived perennials, establishing roots and leafy growth in their first growing season and blooming the second year. Most are hardy in USDA growing zones 3-8, where they do well in cool climates with mild summers. They often reseed, continuing the colony despite living for only a few years.
Optimal growing conditions for the rugged perennials include:
- Full sun to partial shade
- Consistently moist soils
- Coarse, loose soils with good aeration and drainage
- A mulch layer for added protection in cold winter climates
Powdery Mildew on Lupine
Powdery mildew is one of the most common fungal diseases and affects an array of plants, from vegetables to ornamentals. The pathogen Erysiphe presents as a grayish-white coating on leaves and stems. With specialized pathogens like E. lupini, the fungus favors the same conditions as the host.
The white growth on lupine leaves and stems is the fungal spores, and also root-like structures that are sent out to feed on cellular tissues. The white substance starts as single spots that quickly merge to cover the entire surface.
Powdery mildew spores spread by traveling on the wind, contact through tools, or splashing up from the soil via raindrops and irrigation. The spores grow on the leaves and spread to the stems, depleting the energy and nutrition.
In addition to turning white, lupine leaves may turn yellow, brown, and drop during severe infections. New growth may twist and distort. The fungus spreads easily; fortunately, powdery mildew is treatable with early detection.
Cultural Corrections and Prevention
Adjusting growing conditions, along with preventative practices, makes strides in combating powdery mildew in lupines. In severe cases, antifungals added to the mix are a full-scale approach.
Remove Impacted Parts
If lupine leaves show white spots or surfaces, clip or handpick them off if practical. Dispose of the leaves in a garbage bag or burn pile, away from the garden and out of the compost bin, to prevent harboring the spores.
After flowering, the short-lived perennials may show reduced vigor and mildewy leaves. Cut back the white lupine leaves when deadheading the spent blooms. They’ll put a flush of new growth and hopefully a second round of flowers, too.
Best Watering Practices
While powdery mildew doesn’t need moisture to thrive, humid, damp conditions are favorable for its development. And, spores spread by splashing up from the soil during irrigation. When feasible, water at the base of the perennials to avoid wetting the leaves unnecessarily.
Lupines, with their extensive roots, are drought-tolerant, though fluctuations in dry and wet conditions cause stress and lead to pest and disease susceptibility. Provide consistent moisture for best vigor. If using overhead irrigation, water in the morning so leaves dry out in the day’s sun and breeze.
Spacing and Airflow
Ample air circulation is key to staving off fungal problems for lupines. To prevent future problems, provide proper spacing by thinning crowded seedlings. As they reach two inches tall, thin them with spacing according to the mature size of the variety. Thinning ensures proper air circulation, root space, and room for maturity. Thin by clipping excess seedlings at their base rather than pulling and risking root disruption of those remaining.
Proper spacing for lupines is generally about one to two feet apart. Small varieties benefit from a 12-18-inch spacing, while larger ones need two to three feet apart.
Prune excess foliage on mature stems, and trim surrounding plants if they encroach on the perennials.
Increase Sun Exposure
Heavily shaded sites can promote the spread of powdery mildew. The disease doesn’t require moisture to thrive, so even dry shade poses a risk. Grow lupines in full sun, with six or more hours daily, to lessen the chance of infection. They tolerate partial shade (four or more hours of sun), but incidences of the fungus decrease on the sunnier side.
Sunlight and heat, too, can kill off spores. Leaves at 95°F (35°C) show effectiveness in spores dying off, but that’s outside the optimal range for lupines, who prefer milder conditions below 90°F (32°C). The moderate temperatures that lupine enjoys are also the best for powdery mildew.
Avoid Overfertilizing
Lupines don’t need fertilizer to thrive. They tolerate various soil types, as long as they’re well-draining. They grow in lean soils, including sandy or rocky ones.
Excess fertilizer stimulates quick, leafy growth and stems. The weak, fast growth increases lupine susceptibility to powdery mildew. The overgrowth can also foster spore development.
End of Season Cleanup
Good garden sanitation is one of the best ways to prevent those telltale white lupine leaves. After a few heavy fall frosts, the herbaceous perennial dies back to the ground. Foliage turns brown, shrivels, and dries.
To prevent spores from harboring in debris, cut the brown basal foliage to within one to two inches of the crown. Remove the material for a clean site and fresh foundation for the next round in spring. You can even mow lupines planted in a mass or meadow situation to prep the site for winter.
In keeping with good practices, sterilize pruners between uses to avoid spreading the spores among plants when clipping. A 70% isopropyl alcohol bottle or spray makes a quick sanitizer while working, as does an alcohol wipe.
Antifungal Applications
Antifungal sprays are best as a preventative for powdery mildew or to treat it early on. It prevents spread to uninfected plant tissue. Use these as directed, usually once a week or every two weeks, depending on seasonal conditions. Organic fungicides with sulfur offer a preventative and a treatment option.
Potassium Bicarbonate
Organic elements in fungicides like bicarbonate can be practical treatments for powdery mildew on lupines. Sodium bicarbonate disrupts the cell walls of fungal spores. Commercial products are available, or you can try a DIY version using one tablespoon of baking soda in one gallon of water. Add a half-teaspoon of insecticidal soap or castile soap as a surfactant (avoid using dish detergent, which can burn leaves). Make a fresh spray to use every week or two. Avoid overuse, which can accumulate in the soil and burn leaves.
Baking soda applications are best as a preventative measure rather than treating severe infections. If your lupine is already fungus-laden, skip this remedy and go to solutions like neem oil and sulfur.
Milk Spray
For a mild, if smelly, treatment, try milk as a homemade fungal treatment. Milk has surprising antifungal qualities with compounds that are antiseptic against fungal spores. The compounds may prevent outbreaks and support plant resistance against diseases.
Research indicates that higher-fat milk is one to use. For effectiveness, combine one part milk with three parts water (straight milk without water can have side effects)
Combine the milk and water and shake the mixture. Use it as a foliar spray and weekly preventative. A drawback to this easy spray may be a spoiled milk smell in summer’s high heat.
Horticultural Oils
For more extensive outbreaks, horticultural oils like neem or horticultural soaps can treat powdery mildew early in the game. Don’t combine oils if you’ve treated with sulfur within two weeks.
Oils are another option, heavier on prevention than treating full outbreaks, but they’re dual-purpose. They target pests as well as many diseases. Follow application guidelines, as usual, since they impact insects like pollinators and beneficial predators.
