Author:Dave Clement – 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

Why are so many oak trees dying this year?

October 1, 2019September 16, 2022Maryland Grows14 Comments

dead oak tree — *Rapid decline of an oak tree in an area with a restricted root zone. Photo: D.L. Clement, University of Maryland Extension*

This season (2019) the Home and Garden Information Center has received a tremendous number of questions on rapid browning and death of many of our oak trees in urban landscapes and forest situations. Even though it would be convenient to point to a single reason for this dieback it is most likely a combination of weather, disease, and insect factors.

A logical starting place to look for an explanation would be the often-overlooked gradual health decline of our trees due to old age, restricted root zones, soil compaction in work zones, old trunk wounds, storm damage, poor pruning, urban stress such as reflected heat and drought, and opportunistic diseases and insects. These decline factors can extend over many years, leaving trees to try and cope with less than ideal growing conditions.

Last season these conditions were further worsened by the excessive rainfall that continued into this spring which resulted in standing water at many locations that had low spots, compacted soil, or water collection points. Flooded soils and saturated root zones further weakened trees by allowing root pathogens such as Phytophthora a chance to reduce the overall number of healthy roots.

area of flooding near oak trees — *Flooding near oak trees, Spring 2019. Photo: D.L. Clement, University of Maryland Extension*

In general, red, black, chestnut and white oaks don’t tolerate poorly drained soils. Trees can tolerate some reduction in root health, as long as temperatures remain cool, water demands aren’t high, and adequate time is allowed for root regeneration. As a root system loses the ability to support the tree’s water needs, dieback will occur especially in the upper branches.

When the high summer temperatures began this season in mid-July and the low rainfall extended into this fall these conditions accelerated the loss of tree vigor and resulted in sudden browning of tree leaves and canopy dieback. Compromised tree health often allows pathogens such Armillaria and Hypoxylon to invade, which further accelerates dieback and death. In addition, opportunistic insects such as Ambrosia Beetles and Two-lined Chestnut Borer, will attack tree trunks and continue tree demise.

frass on oak from ambrosia beetles — *Evidence of a boring insect infestation. Photo: D.L. Clement, University of Maryland Extension*

There are a few positive steps that may alleviate some tree stress. It is very difficult to reverse decline in stressed oaks so select trees that still have green foliage and irrigate near their bases during this period of high drought stress. Even minimal amounts of water can help recovery and prevent drought stress before winter dormancy. Practices that open up compacted soils to increase drainage and raise soil oxygen levels (e.g., vertical mulching) will often help as well.

As we continue to receive information about dying oaks across the state, we still have many unanswered questions. We will continue to collect data on tree species, age, and pest occurrence, in coordination with other agencies across Maryland.

By Dr. David L. Clement, Principal Agent, University of Maryland Extension, Home & Garden Information Center and Dr. Karen Rane, Director, University of Maryland Plant Diagnostic Laboratory

Boxwood Blight in Maryland

April 1, 2019June 15, 2021Maryland Grows2 Comments

Until last summer most people in Maryland weren’t aware of the new fungal disease infecting boxwood called boxwood blight. In 2011 professionals in the green (landscape and greenhouse) industry were informed of the disease but the outbreaks were scattered and insignificant. However, the rainy 2018 season greatly increased the spread of the disease. It has now become more noticeable in Maryland landscapes. In addition, on a few occasions, it has been observed on Japanese spurge (Pachysandra terminalis) in Connecticut and on sweetbox (Sarcococca sp.) in Maryland and Virginia. Essentially, boxwood blight occurs up and down the east coast.

Boxwood blight will infect all boxwoods grown in landscapes. However, some cultivars, especially English and American, are more susceptible than others. See the following photos for symptoms of boxwood blight.

boxwood blight leaf lesions — *Dark leaf spots are a symptom of boxwood blight. Photo: Dave Clement*

boxwood blight stem lesions — *Narrow black lesions (cankers) on green stems are a key symptom of boxwood blight. Photo: Dave Clement*

boxwood blight lesions on dead stems — Black lesions on stems of boxwood. Photo: Dave Clement

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So the question is what to do if your shrubs are diagnosed with boxwood blight? The best information for homeowner action is located on the Virginia Boxwood Blight Task Force website: Best Management Practices for Boxwood Blight.

Here is a quick summary of what to do in the landscape:

A strong suggestion is to avoid planting any new boxwood plants in your existing landscape or bringing in boxwood greenery, including holiday boxwood wreaths.
If planting, inspect your plants carefully and ask if the plants have been raised in a certified “cleanliness program.”
Observe and watch any newly planted boxwoods carefully for disease symptoms.
Send photos of suspicious symptoms to the Home & Garden Information Center’s Ask Extension service.
If disease symptoms are diagnosed, immediately bag and remove infected plants along with fallen leaves. Mulch the area to bury remaining debris. Do not compost infected boxwood material. Launder all clothing, gloves, and shoes, and sanitize gardening tools. Removal will not guarantee eradication of the boxwood blight pathogen since it can survive in resting structures in the soil for many years.
Fungicide sprays have shown some disease suppression in limited situations. However, these treatments do not eradicate boxwood blight and need repeated applications throughout the growing season.
Consider replacement of boxwoods with non-susceptible plants such as hollies and conifers.

By Dr. Dave Clement, Principal Agent, University of Maryland Extension, Home & Garden Information Center.

Japanese Maples in Maryland Landscapes: Plant Location & Care Are Keys to Success

August 24, 2018November 17, 2021Maryland Grows6 Comments

Japanese maple leaves — *Japanese maple. Photo: Pixabay*

The group of small ornamental shade trees lumped under the name Japanese maples, Acer palmatum and A. japonicum, and their many hybrids, are very popular with gardeners and plant enthusiasts. Most of the questions we receive about problems with Japanese maples are horticulturally related to poor growing conditions and maintenance rather than insects or diseases. The causes of these problems are usually root or trunk-related issues. So, let’s start with a look at the planting conditions Japanese maples need in order to thrive. Continue reading →

Why Is Leyland Cypress Turning Brown? Winter Took Its Toll

April 11, 2018April 7, 2021Maryland Grows3 Comments

Leyland cypress showing winter damage — *Winter damage on Leyland cypress trees. Photo: Dave Clement, University of Maryland Extension*

Leyland cypress (x Cupressocyparis leylandii) has a lot going for it. It’s fast-growing with evergreen, feathery foliage and a pleasing, slender profile. It makes an excellent specimen tree or screening plant. A cross between two Pacific coast species, the Leyland cypress thrives best in moist, cool climates with moderate temperatures. These trees are hardy to zone 6, however, they do not tolerate sudden temperature fluctuations. We indeed experienced some of these very cold sudden temperature fluctuations this past December and January. The first winter damage symptoms will begin showing up as browning and dieback this spring as temperatures begin to warm and stimulate new growth.

Continue reading →

What’s Wrong with My Blue Spruce Tree?

January 8, 2018April 12, 2021Maryland Grows11 Comments

blue spruce with cytospora — Blue spruce with symptoms of Cytospora canker.

Dr. Dave Clement, University of Maryland Extension Plant Pathologist, explains two common diseases of this popular evergreen tree.

Colorado blue spruce trees, although not native or adapted to Maryland, are commonly planted in landscapes for their attractive color and shape. There are, however, two plant diseases that commonly infect and disfigure them. Both diseases also can occur simultaneously and progressively speed up the decline of this popular tree.

Continue reading →

Why are Oaks Declining? Dr. Dave Describes 3 Oak Diseases

September 11, 2017April 27, 2021Maryland Grows3 Comments

Leaves showing oak wilt symptoms, Pittsburgh, PA (Photo: Dave Clement)

We’ve recently received inquiries about oak trees declining and dying. People usually want to know if this is caused by oak wilt. Importantly, oak wilt is not known to occur widely in Maryland. Two common leaf diseases of oaks in Maryland are oak anthracnose and bacterial leaf scorch of oak.

Oak anthracnose is caused by a fungal pathogen, Apiognomonia quercina, and it typically is a cool, wet weather, springtime disease. In wet cool summers like the one we’re experiencing this year however, the symptoms can continue through the season. Most oak species will show a range of symptoms scattered through the leaf canopy. Symptoms range from small brown spots and irregular dead areas on distorted leaves to severe blight that kills twigs and causes leaf shriveling. This disease in oaks typically doesn’t cause much leaf drop.
Continue reading →