Autumn has arrived in Wisconsin, with days shortening and temperatures cooling. As gardeners begin putting their gardens to bed for the winter, they have been coming across what they consider alarming developments on their herbaceous ornamentals, deciduous trees and evergreens. Luckily, most of what they have been seeing is innocuous and non-life-threatening. Here is a rundown of some of the issues that I’ve been hearing about.
Herbaceous ornamentals. Many clients have commented on an uptick is what appear to be foliar diseases on a wide range of herbaceous perennials. While I have certainly seen a number of fungal and bacterial diseases on herbaceous plants over the course of the summer, much of the dieback I have been seeing in September and October has been natural dieback as plant start to go dormant for the year. As the days shorten, perennial plants start moving nutrients from leaves and into crowns and roots where these nutrients can be stored for the winter. Leaves yellow and brown as a consequence of this nutrient movement. These changes can occur quickly and look very dramatic and disease-like, but this is normal for this time of the year.
From a disease standpoint, I like to point out that as plants go into “winter mode” at this time of the year, pathogens do as well. For plants with phytoplasma diseases like aster yellows, as these plants transport nutrients into their roots and crowns, they also concentrate phytoplasmas in these tissues, where the organisms overwinter. Powdery mildew fungi often overwinter as hyphae (i.e., fungal threads) in the overwintering buds of perennial plant hosts. Above-ground plant debris is another place where a variety of plant pathogens can survive the winter. For that reason, I routinely emphasize the importance of garden cleanup to remove these materials and eliminate a source of fungal spores that can infect next year’s plants. Most gardeners traditionally do cleanup in the fall, but there can be reasons (e.g., improving winter appeal of a garden, leaving overwintering sites for important plant pollinators) for doing this cleanup in the spring. Cleaning up before new leaves emerge in the spring is critical however, for good good disease control. Burning (where allowed), burying or hot composting are typical ways of disposing of old plant debris.
Deciduous trees and shrubs. Leaf diseases on trees and shrubs have been quite prevalent this past summer, but as gardeners have begun to rake leaves, one particular disease, tar spot, has been generating a number of questions for the PDDC. Tar spot is a fungal disease, characterized by formation of black, tarry spots on leaves of maples. These spots appear to be more visible (and thus disconcerting to gardeners) in the fall, most likely because the spots are more easily visible against leaves that have turned bright fall red or yellow, compared to the dark green of leaves in the summer. There are two variations of tar spot that occur in Wisconsin. On native silver and red maples, the tar spot fungus is Rhytisma americanum, which causes large, solid black, raised areas that look as though someone left a thumbprint in the middle of the tarry spot. On Norway maple (a European maple species), the tar spot fungus is Rhytisma acerinum, a non-native fungus that causes large, diffuse (spotted-looking), flat, black areas. Both types of tar spot are cosmetic. Good cleanup of the infected leaves (as described above) should provide adequate control of the disease.
Evergreens. I have recently been getting numerous questions about yellow or orange/brown needles on white pines and arborvitaes (as well as occasionally on other evergreens). The discolored needles are typically very vibrant in color and are, for the most part, interior, older needles. The timing of the color change (September into October), as well as the intense color and location of the affected needles, points to this being something called seasonal needle drop. Seasonal needle drop is a natural needle color change and loss that evergreens can go through in the fall. It is equivalent to the color change that we are used to seeing on broad-leaved trees (like oaks and maples) every autumn. While sometimes dramatic, seasonal needle drop is normal and not detrimental to trees and shrubs.
Are you seeing what you consider alarming developments in your garden or landscape? Hopefully not, but if you are, and need help diagnosing these problems, feel free to contact the PDDC. For the PDDC’s current policy on sample submission, including submission of digital photos, check out the following link. As always, be sure to check out the PDDC website for timely information on plant diseases. Also, feel free to follow the clinic on Twitter or Facebook (@UWPDDC) to receive timely PDDC updates. Or alternately, put in a request to subscribe to the clinic’s new listserv (UWPDDCLearn) by emailing firstname.lastname@example.org.
Hang in there, be safe, and stay healthy everyone!