Dendro and and his assistant Codit were asked to visited a wedding venue to examine defoliating Viburnum suspensum. They met its grounds manager, Amelia. Several of those shrubs are near a gazebo, and she expected that the background should be green and suitable for wedding photos. Since last year, there was defoliation and leaf spotting on one shrub, and that spread to others. Their landscape contractors assumed it was a leaf spot disease and sprayed a fungicide three times for one year, but there was no improvement. In the middle of their conversation, the pop-up sprinklers activated and sprayed water onto their pants and shoes. Amelia apologised, as the irrigation should have been turned off during the meeting, but Dendro and Codit commented that cultural conditions are important. Upon further questioning, Amelia shared that the shrub bed is irrigated every other day, and long enough to soak the grass and mulch. Dendro replied that foliar pathogens are spread and supported by excessive moisture.
Codit examined the leaves on the ground and noticed that the spots on them are angular. "Fungal leaf spots are often medium to dark brown and more rounded, while bacterial spots are generally dark brown with a yellow halo", he explained further, "angular leaf spots are less common". Foliar nematodes and plant pathogenic bacteria can cause angular leaf spots. Dedro recommended sending samples to plant diagnostic laboratories to know for sure the pest and disease issues, or health problems, in order to be sure.
Upon inspection on a leaf with a magnifier, Codit spotted grayish fuzz growing on spotted tissue and deduced that the disease is viburnum downy mildew, Peronospora viburni. Downy mildew diseases are caused by oomycetes, which are fungus-like organisms, not true fungi. Diseases caused by oomycete are usually worse when conditions are wet, as they produce spores that swim in water. Phytophthora species is an oomycete that can infect foliage as well, but its often seem on roots, and stem/trunk cankers on woody plants. Downy mildews are often host-specific, so one plant genera is being infected by one species of downy mildew - Viburnum suspensum shrubs in this case.
Amelia connected the dots and realised that the reason why the landscape contractor's fungicide applications were not working was because the disease is not a fungus. Dendro reminded Amelia to send leaf samples to a diagnostic lab, and inform the landscape crew to rake up and dispose of gallen infected leaves regularly. In addition, to turn the sprinklers away from the viburnum foliage so that they are not wetted. Finally, to do scouting so as to catch disease and insect problems early.
Reference: Arborist News, Apr 2025, pages 22, 23, 56, 57
