Diseases and Insects Associated with a Wet Summer

The East Coast experienced one of the wettest summers since 2003 and 2004, when new rainfall records were set. The interesting factor this year was the frequency of the rainstorms; just when things would dry out we would get another downpour. The silver lining in these dark clouds was that most cut flower growers had a reduced need for  irrigation. With these frequent rains, many cut flowers flourished, but so did weeds, diseases, and certain insects.

Foliar diseases such as bacterial leafspot, Alternaria, and Cercospora leaf spot on zinnias were common in 2013.  Unless you were applying a lot of foliar fungicide and bactericide sprays it would have been nearly impossible to avoid these diseases in such a wet year. Extended periods of wet foliage and warm, humid air were ideal incubators for disease. High moisture levels in the soil created conditions perfect for root rots.
Spacing plants for optimal air circulation helps reduce foliar disease. Improving soil structure to encourage better drainage lessens the prevalence of root rots.

One disease commonly seen on sunflower was downy mildew. Many different races of this obligate pathogen exist, and they can be cultivar specific.  Downy mildew of sunflowers is caused by the soil-borne fungal-like pathogen Plasmopara halstedii.

The disease is most active during cool, wet growing seasons, and can stunt, and in some cases, kill plants. Systemic infection of sunflower seedlings results when zoospores infect the roots.  Plants that survive this initial infection are dwarfed and chlorotic. White zoosporangia appear on the underside of the chlorotic areas of leaves. 

Secondary infection is not as serious. It’s most common when sunflower foliage remains wet for prolonged periods, and windborne spores land on the leaves, producing angular chlorotic lesions.

Systemically infected plants usually are severely dwarfed. One mistake growers make is plowing infected plants into the ground. The pathogen can survive for up to ten years in soil as thick-walled resting structures called oospores.  Oospores are produced in the tissues of infected plants and are more common in roots than leaves.  Wild sunflowers as well as some weeds like marsh elder are also hosts for this pathogen. Spores are also wind blown, so even fields with no previous sunflower history can become infected during wet seasons.

In the upper Great Plains where sunflowers are grown commercially for oil production, fungicide seed treatments are frequently used at planting.  Most cut flower producers don’t apply fungicides, so best management practices should include quick removal of infected plants from the field followed by disposal. Avoid planting sunflowers in wet fields or low areas. There is anecdotal information from cut flower growers that some cultivars of sunflowers are less susceptible than others. This might be a good field trial to conduct in the near future. We would love to hear your feedback if you are finding certain cultivars more susceptible or less susceptible. This will help give us a baseline of sunflower cultivars to evaluate.

A Rainy 2013 Means a Buggy 2014

The rains also affected the insect world. High soil moisture levels in July and August in 2012 resulted in high survival of larvae of scarab beetles. Adult scarab beetles such as Japanese beetles were very abundant in Maryland this summer feeding on roses,  dahlia, and zinnias in many cut flower operations. The wet weather from July through August enabled many of the larvae of the beetles to survive, so we can expect a high level of beetle pressure in 2014. Growers on the East Coast might want to plan a scarab beetle strategy for next season.

Several insecticides are labeled for beetle control, including carbaryl (Sevin), acephate (Orthene), and several pyrethroids. The problem is that many of the “older chemistry” insecticides can kill, sicken or repel beneficial insects, spiders and mites. One presently labeled low-risk material for Japanese beetle control is neem. Neem (Neemix, Azatin, Aza-Direct) is a botanical insecticide that contains azadirachtin, which has minimal impact on beneficial organisms.

In trials conducted at the Central Maryland Research and Education Center in 2005 we evaluated neem products, as well as a synthetic pyrethroid (Permethrin) for pest control on zinnia plants. The permethrin treatments gave the best control for 5-7 days. Adult Japanese beetle populations were at an extremely high level in 2005 and feeding pressure was very heavy.  Japanese beetles inflicted damage to zinnias receiving all treatments before being impacted by the chemical treatment. The azadirachtin (Neemix) treatment appeared to provide repellency to beetle feeding but appeared to have short residual activity of only 3-4 days. Therefore 2 additional applications of azadirachtin were applied during the trial to reduce damage by Japanese beetles. When Japanese beetle population pressure is high, any grower can expect some level of  damage even with the most effective insecticides.

We are conducting trials to evaluate damage from brown marmorated stink bugs on cut flowers in 2013. This past summer’s heavy rain tended to move the nymphs off the foliage and stems temporarily but when the heat and humidity increased, they returned rapidly. The nymphs are fast and move around finding new feeding sites. The interesting thing is the nymphs do not fly but they will walk several meters to find plants to feed on. We found the nymphs swarming over gladiolus, feeding on foliage and unopened flowers in mid-August. Damage was apparent on foliage and unopened flowers. The growers allowed us to take data on the gladiolus for two weeks before they felt they had to treat the plants. We will have a summary of our trials in an upcoming issue.

A Bit of Good News

Several species of caterpillars did extremely well this season with the abundant supply of foliage used for food.  Since many of these caterpillars were not species that feed on cut flowers, life was good. A positive result of the high caterpillar numbers, especially different swallowtail species, was that many cut flower growers saw a significant increase in the number of butterflies and skippers visiting their fields. Several growers commented they could not remember seeing so many butterflies in their operations. Zinnias, crocosmia, cosmos, any of the composites, butterfly bush, and Eupatorium purpureum (Joe Pye weed) are highly attractive to butterflies.

Finally, if you could figure out a way to market weeds, 2013 was perfect for you. Growers had a real battle keeping weeds down last summer. As soon as they were killed, more popped up to take their place, nourished by the unending rains.

We cannot control weather but we think most growers would prefer a dry summer and a healthy trickle irrigation supply.

Monarch butterflies were slightly more populous in some mid-Atlantic states in 2013

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]

David Clement

Extension Specialist in Plant Pathology

David Clement is Extension Specialist in Plant Pathology, University of Maryland Extension. Contact him at [email protected]