downy mildew – Maryland Grows

Basil Bounces Back With Downy Mildew Resistant Cultivars

February 21, 2020February 17, 2020Maryland Grows5 Comments

Basil downy mildew arrived in the U.S. (Florida) in 2007 and has been devastating Italian sweet basil crops in Maryland and other states ever since.

The disease (technically a water mold) can only survive on live basil plants so it does not overwinter outdoors in Maryland. The infection moves from southern states northward each summer. Infected leaves turn yellow between major veins and this symptom eventually spreads across the leaf. The characteristic sign of the pathogen appears as a fuzzy grayish-purple coating (sporangia) on the lower leaf surfaces. Infected leaves eventually turn brown and plants collapse during warm, humid weather.

Spore structures (sporangia) of basil downy mildew on leaf undersides.

Resistance to the Rescue!

Thankfully, breeders have been busy developing resistant cultivars for this popular crop. Over the past two years Rutgers University has released four cultivars, ‘Obsession,’ ‘Devotion,’ ‘Passion,’ and ‘Thunderstruck’. An Israeli breeder developed ‘Prospera’ and Proven Winners came out with ‘Amazel,’ which is seed sterile and propagated from cuttings- so only available as plants.

Trials in Maryland

A limited field evaluation of four basil cultivars- three with reported resistance and one susceptible cultivar- was conducted during the 2019 growing season at three Maryland sites: Westminster, Finksburg, and Ellicott City (Central MD Research & Education Center).

Plants were established in the fourth week of May. Field observations of downy mildew symptoms were noted weekly. Plants were rated on a score of 0 – 10 with zero being free of disease. Disease ratings below were combined from the three sites.

BASIL	August 10, 2019	Sept 3, 2019	Oct 7, 2019
‘Amazel’	0	0	0
‘Prospera’	0	0	0
‘Devotion’	0	1	2
‘Obsession’	0	1	2
‘Aroma’ (susceptible cultivar)	5	9	10

Basil cultivars left to right ‘Amazel’, ‘Aroma,’ and ‘Prospera’ (8/15/19)

Basil cultivar ‘Obsession” with mild downy mildew symptoms (9/5/2019)

Summary

Disease symptoms were first noted on August 10 on ‘Aroma,’ and progressed rapidly. Observations of the resistant cultivars continued through October 8. By September 3^rd, downy mildew had caused very faint yellowing on the lower leaves of ‘Devotion’ and ‘Obsession’. In general, all the resistant basil cultivars performed quite well compared to the susceptible cultivar ‘Aroma’. Mild disease symptoms on the Rutgers cultivars ‘Devotion’ and ‘Obsession’ did not progress; plant damage was very minor. Prospera’ and ‘Amazel’ never developed disease symptoms during the trial.

HGIC also received positive reports from UME Master Gardeners and other gardeners about the productivity and resistance to downy mildew of ‘Prospera,’ ‘Obsession,’ and ‘Devotion’.

Seed Availability

‘Devotion,’ ‘Obsession,’ and ‘Prospera’ seeds are sold by Johnny’s Seeds and High Mowing Seeds. Harris Seeds is carrying ‘Prospera’ seeds.

Other resistant cultivars: ‘Pesto Party’ has shown limited suppression of basil downy mildew (seeds available from Burpee Seeds). Be on the look-out in 2021 for the other new cultivars from the Rutgers breeding program, ‘Thunderstruck’ and ‘Passion,’ that show very good resistance to downy mildew.

Resources:

http://blogs.cornell.edu/livegpath/research/basil-downy-mildew/

https://plant-pest-advisory.rutgers.edu/an-introduction-to-rutgers-downy-mildew-resistant-sweet-basils-2/

By Jon Traunfeld and Dave Clement, Ph.D., Extension Specialists, UME

Basil downy mildew is back in Maryland gardens

July 20, 2018July 17, 2018Maryland Grows1 Comment

Hurry up and enjoy your basil plants while they are healthy. That insidious disease, downy mildew, is infecting basil across the mid-Atlantic region. I first noticed it in a Howard Co. community garden last week and then in my home garden on July 15^th.

Caption: Early, subtle symptoms — Early, subtle symptoms

The initial symptom is splotchy light yellowing of leaf topsides. Those areas eventually turn brown and die. Leaf undersides develop a fuzzy grayish fungal mat with spore structures that appear as tiny dark specks.

The disease spreads rapidly. You can cut back infected plants to encourage new growth but the young leaves quickly become infected. Planting multiple times in different locations can be a useful strategy. Once plants are infected it’s best to pull them out, harvest usable leaves, and trash the rest of the plant.

The good news is that a Rutgers University team developed four new resistant cultivars available to commercial growers this season. Hopefully, there will be plenty of seed harvested this year so that we can all enjoy the benefits of these new releases.

By Jon Traunfeld, Extension Specialist

Squash Family Pest Problem Tips

April 20, 2018August 23, 2022Maryland Grows3 Comments

The squash plant family (Cucurbitaceae) includes many garden favorites- cucumber, summer and winter squash, pumpkin, muskmelon, and watermelon. Unfortunately, it’s a family vulnerable to some of the most consequential insect pest and disease problems. The names squash bug, cucumber beetle, squash vine borer, downy mildew, and bacterial wilt strike fear in a gardener’s heart. And those just represent the starting team. The legion of potential pest problems is sufficient to bring the toughest gardener to his knees sobbing in anguish.

But there’s hope for the human animals competing against insects, mites, and pathogens for these valued food resources. There are many ways to prevent and manage these problems and these are covered in detail on the HGIC website. Here are a few strategies that I think are less widely used. Give them a try in your pursuit of higher yields with fewer tears!

Select disease resistant cultivars. Cornell University’s Vegetable MD Online website has a matrix for each vegetable crop that lists all major diseases and cultivars with resistance claims. They even include seed companies that sell the seeds! Special note: ‘County Fair’ cucumber is resistant to bacterial wilt; butternut and ‘Tromboncino’ squash are fairly resistant to squash vine borer.
Apply floating row covers when seedlings emerge or transplants go in. The material excludes all critters. Remove the cover when plants start to bloom.
Plant around insect pests by planting healthy transplants as soon as conditions allow or waiting until mid-June to plant seeds. Plant pumpkin and winter squash from late June to July 4^th.
Keep planting. Cucumber and summer squash can be sown several times, 2-3 weeks apart.
Scout your plants for signs and symptoms of problems and take action early!

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