Beardtongue leaf disease
I have weird dimpled red spots that are affecting my beardtongue plants (both husker red and the green leaf kind). By the beginning of June last year, almost all the leaves had spots (see photos of whole plants). So far this year, I'm seeming some spots, but it's not all over (yet). The close up photo is from this year and shows the underside of the leaf. I'm worried that this will spread, and it will be just as bad as last year. I've tried searching online, and I can't tell what's the problem. Can I just remove the infected leaves? If I should use a spray, I'd like to as organic an option as possible. These plants are in my pollinator garden. The beardtongue seem to be the only ones affected. Thanks!
Ramsey County Minnesota
Beardtongue seems to be susceptible to rusts. Rusts, named for their distinctive red-orange colored spores, are fungi that cause plant diseases and can be seen from spring through fall on infected plants. Keep an eye out for bright colored rust spores in May or June after a warm rain.
All rusts are obligate parasites, that is they require a living host to complete their life cycle. Different species of rust fungus can go through up to five separate spore stages during a life cycle that requires two or even three different host plants. Bright red urediniospores are produced in vast numbers and spores from this stage can be blown on air currents for hundreds of miles to infect new plants. Urediniospores require moisture to germinate but when that happens a spore that lands on a leaf will physically punch into otherwise healthy leaf tissue. Tough dark-colored teliospores overwinter in debris and infect plants early in the growing season.
Many strategies can be done collectively to reduce the disease in following years. The most important is cleanup in the fall.
Rake up and destroy fallen leaves before the first snowfall to eliminate locations where pathogens can survive to re-infect the plant the following growing season.
Do not overcrowd plants
Thin to increase light penetration and improve air circulation
Wet conditions promote disease, so avoid or redirect lawn and landscape sprinklers.
Fungicides are protective and need to be applied before symptoms appear on the leaves. Proper timing of fungicide applications can vary depending on the biology of the disease.
If you choose to use a fungicide next spring, read the label to be sure it is appropriate for beadtongue and apply as plants emerge