Verbenas have a key position among bedding, hanging basket and container crops. They provide pure, bright colors or delicate, pastel shades and generally grow like gangbusters. This innate vigor protects them from some of the disease problems that require a debilitated host. Yet, there is one pathogen group that takes advantage of perfectly healthy plants, those that are not stressed in any way: the powdery mildews (PM). Because PM caused by Podosphaera xanthii has been especially troublesome to manage on verbena in recent years, we have focused on it in an American Floral Endowment-funded research project that aims to improve PM management for ornamentals.
Podosphaera xanthii (previously known as Sphaerotheca fuliginea) is a PM with the ability to infect more than one kind of plant. It is most familiar as a disease of cucurbits, the group of vegetables that includes cucumbers, squash and pumpkins. In those crops, PM is harmful because of the deleterious effects on the foliage — when heavily infected, leaves may turn brown and shrivel on the vine, leading to substandard, sunburned fruits. Growers who produce cucurbit transplants as well as verbenas should be especially careful to separate these two crops. Verbenas over wintered in the greenhouse have been observed to introduce PM to seed-grown squash and cucumbers that would otherwise not have encountered infection until after several months of field cultivation.
Of course, carrying some plants over from the previous season may introduce PM to spring crops of verbena as well. And each cutting source may inadvertently share some powdery mildew inoculum: Traces of PM can hitchhike unnoticed from onshore or offshore production sites along with cuttings. Although PM is obvious when it is well established, it can reside in small colonies on the undersurfaces of leaves that are not detectable until conditions allow the development of an epidemic that magnifies the visible signs of the disease.
Powdery mildew on verbena typically begins as thin, whitish patches on the underside of lower leaves. Eventually these leaves show the ill effects of the disease by developing a yellow or purplish cast. Leaves with this appearance should be turned over to check for PM underneath. The symptoms can easily be mistaken for nitrogen deficiency.
This ability of PM to be unobtrusive until temperature and humidity conditions trigger dramatic spread is the reason why monitoring your crop is so critical. Inspecting incoming cuttings and the growing crop on a regular basis is important to catch the first signs of an infestation. Fungicides work most effectively as protectants, preventing successful establishment of new fungal colonies. The highest crop quality is maintained when fungicides are used before the PM has spread significantly.
After noting that growers saw a great deal of variation in the amount of PM damage on different cultivars, we decided to explore the genetic variation in some of the verbenas available in the trade today. We have conducted three trials to date, two in greenhouses and one in a garden setting. These experiments have allowed us to observe the disease susceptibility of 125 cultivars in at least one set of environmental conditions.
Just as growers have noted, we have seen wide variation in the performance of different cultivars. We have not yet accumulated enough information to give each cultivar a specific rating relative to the others available in the trade. However, we have observed that some cultivars appear to be especially prone to PM and develop unsightly foliage under normal growing conditions. The cultivars that have shown a high level of symptom development in at least one of our trials are listed in Figure 1, page 34.
Growers who choose to grow these cultivars should anticipate that PM is likely to develop. They should either scout these cultivars very carefully for the first symptoms, paying attention to the undersides of lower leaves, or consider using fungicides preventively.
We certainly have not tested cultivar performance enough times in enough different environments to be confident that any verbena cultivar is entirely resistant to PM. However, a number of cultivars that have been tested at least twice have thus far shown very low susceptibility to PM (see Figure 1, page 34).
Our studies have convinced us there is a wealth of genetic variation available in verbenas, such that no grower should need to be burdened by intensive disease management for a highly PM-susceptible cultivar year after year. Often plants within the same series may show different PM susceptibility, so judge each cultivar on its own merits. Fungicide application frequency can be significantly reduced by choosing to grow plants that are less susceptible to disease.