Early season disease problems to scout for in ornamental nurseries
Nursery growers should watch for fungal and bacterial blight problems with warmer days and cool nights on woody ornamental plants in polyhouses.
Nursery stock continues to be stored and shipped out of polyhouses at this time. If pest problems are identified, growers should use caution when applying pest control sprays. Temperatures in the polyhouse can spike on clear sunny days, causing the insecticide or fungicide to injure foliage. Currently, I see ventilation holes cut to keep high temperatures and high humidity from occurring as the common response by growers in the area. Research has shown that high humidity has been linked to distortion and stunting of new growth on vines and shrubs and should be addressed where these symptoms have been observed. Some fungal and bacterial blights (e.g. Pseudomonas blight on Syringa) of woody ornamental nursery stock can often be attributed to stressful conditions experienced under poly and also the extreme shifts in temperature once the poly is removed. Pseudomonas bacterial blight looks very similar to low temperature injury, and often the two conditions go hand-in-hand. A nice fact sheet to help you identify the symptoms with photos is available from Cornell University.
Nursery growers should be on a routine copper fungicide program as buds begin to swell to help reduce the spread of bacterial blight, especially when lilacs are overwintered in polyhouses. Research also indicates that the copper becomes more effective if combined with Dithane. The bacteria overwinter next to the buds and can infect leaf tissue once bud caps split open.
Prune out and destroy any infected shoots as soon as symptoms appear. Be sure to disinfect cutting tools between cuts. Overhead watering increases the spread of bacterial blight and condensation in polyhouses and can also spread the bacteria on to the plant.
Be aware that freezing temperatures in polyhouses that have plants leafing out can also increase the risk of bacterial blight. If irrigation for frost protection is necessary, be sure to apply a copper fungicide within 12 hours prior to irrigating for frost protection.
Again, infection and spread of this bacterial disease can be reduced where temperatures and humidity levels are moderated (i.e. ventilation under poly) and new foliage is more gradually hardened off to outdoor conditions. For more information contact your local MSU Extension nursery educator.