Two of Our Least Favorite Fall Pest Problems May Be Consorting Together: Fruit Rot and Spotted Wing Drosophila in Fall Berries

Cassandra Swett, Assistant Professor and Extension Specialist – Berry Pathology, University of Maryland and Penn State University; clswett@umd.edu and Kelly Hamby, Assistant Professor and Extension Specialist – Sustainable Agroecosystems and Integrated Pest Management, University of Maryland; kahamby@udel.edu.

Warm rains in the spring and some rains and heavy dews in late summer have made this a bad year for insects and fruit rots, and we continue to see problems going into fall in berry crops. The main fruit rot issues are currently Botrytis in raspberries, and both Botrytis and sour rot in grapes. We have reason to suspect that insects, particularly spotted wing drosophila, may be contributing to the severity of fruit rot damage we are seeing this fall.

BotrytisRaspandGrape

Botrytis fruit rot of raspberries and grapes.

Botrytis impacts are not typically severe unless you get a lot of infested fruit, which can provide secondary inoculum to infect new fruit. We manage for this by applying bloom-time sprays to protect flowers (which are the most susceptible) and reducing humidity in the canopy through thinning, but this will not control the disease if berries become wounded during an infection period, since the pathogen easily enters fruit through wounds. Sour rot in grapes is primarily a wound-mediated pathogen, and wound-management is one of the primary strategies to manage this disease. Wounds can be caused by mechanical injury (rubbing, sun damage) or by insects. In small fruits, spotted wing drosophila (SWD) is one new candidate on the scene that is capable of wounding fruit. SWD makes wounds as it lays its eggs, and this may facilitate fruit rot development if not kept in check. For a review of SWD management, please see the recent special alert article (special alert #2—July 10, 2015) in the UMD Vegetable and Fruit Headline News.

SourRotGrape

Sour rot of grapes.

We are conducting studies at the University of Maryland to evaluate whether SWD activity can facilitate outbreaks of Botrytis fruit rot in fall raspberries and facilitate both Botrytis and sour rot in grapes in the Mid-Atlantic. In raspberries, we are trapping SWD at moderate to high levels, we have confirmed larval infestation in fruit, and we have found SWD larvae in Botrytis infected fruit, indicating that SWD may initiate fruit rot development. Studies are underway to better understand the importance of this potential association.

SWDBotrytisBerry

SWD larva in Botrytis-infected raspberry fruit (arrow).

In grapes, studies in other regions indicate that SWD can initiate sour rot development, and may play a similar role in facilitating Botrytis fruit rot. SWD was present and moderate to high levels in vineyard monitoring trials this year, but we are currently working to see if we can confirm larval infestation in grapes. SWD is likely only able to attack very thin-skinned grape varieties (penetration force of less than 40cN). Botrytis and sour rot are just starting to appear in most vineyards our region, and we will be looking to see if there is an association with SWD.

For more information and updates, please refer to the UMD Berry Pathology Twitter site: https://twitter.com/berry_pathology.

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