Downy Mildew Resistance
- by Todd C. Wehner and Adam D. Criswell
- Department of Horticultural Science
- North Carolina State University
- Raleigh, NC 27695-7609
Cucurbit downy mildew, caused by the pathogen Pseudoperonospora cubensis, is a disease of worldwide importance. Growers in North Carolina and the southeastern United States interested in producing processing or fresh-market cucumbers will need to be aware of the epidemiology and methods of control for this disease. Historically, downy mildew was the most important disease on cucumber in the southeastern United States. Before the 1940’s cucumber growers could only reliably harvest a spring planted crop using available cultivars such as ‘Straight 8’. During the 1940’s a source of resistance found in cucumbers from China was bred into ‘Palmetto’, the first resistant cucumber cultivar available in the United States. ‘Palmetto’ allowed successful production of a summer planted cucumber crop in addition to a spring crop even though disease problems tend to increase from April to October. Unfortunately, this resistance did not last more than a couple of years and the pathogen was able to severely infect ‘Palmetto’ as well as other cucumber cultivars.
During the 1960’s a new source of downy mildew resistance from cucumbers in India was bred into popular commercial cultivars such as ‘Pixie’ and ‘Poinsett’. This new resistance was very effective, once again allowing a second cucumber crop to be grown in the southeast without the use of fungicides. Many cucumber cultivars have since been developed with high resistance to downy mildew. However, in 2004, downy mildew was able to infect even the most resistant cucumber cultivars and has once again become one of the most economically important diseases for North Carolina and many other states. Downy mildew now causes more severe damage on susceptible cultivars such as ‘Straight 8’ and ‘Wisconsin SMR 18’. Resistant cultivars are still resistant, meaning they are significantly less damaged than the susceptible cultivars yet downy mildew causes much more damage than it did previously.
Above is a photograph of a susceptible cultivar (left) compared with a resistant one (right). Click on the image to see a larger version.
Compared to susceptible cultivars, resistant cultivars get the disease later in the season, have less vine stunting, fewer and smaller lesions on the leaves, and less chlorosis (yellowing) of the margin of the disease lesions (Table 1). So far, we have not identified any cultivars that are immune to downy mildew. Most processing and fresh-market cucumber cultivars available from major seed companies for use in the U.S. are resistant to downy mildew. Examples of resistant cultivars for processing are Cates (Nunhems), Fancipak (Seminis), and Calypso (NC State Univ.). Examples of resistant cultivars for fresh-market are Fanfare (Seminis), Poinsett 76 ( Cornell Univ.), and Panther (Nunhems).
Table 1. Cultivar resistance to downy mildew on cucumbers.
| Downy mildew symptoms |
Susceptible
|
Resistant
|
| Disease noticeable during season |
Early
|
Late
|
| Vine size (length) |
Stunted
|
Near-normal
|
| Number of leaf lesions |
Many
|
Few
|
| Size of leaf lesions |
Large
|
Small
|
| Margin color of leaf lesions |
Yellow
|
Green
|
| Example cultivar |
Straight 8
|
Poinsett 76
|
Downy mildew is an obligate parasite; it can only survive and reproduce on living host tissue. Therefore, in most cucumber growing regions of the United States, the disease must be introduced each year from warmer climates. Typically, the pathogen overwinters in southern Florida and Texas on wild or cultivated cucurbits (cucumbers, melons, watermelons) and is spread north in wind and storms. There have been reports of downy mildew overwintering in greenhouse operations in northern climates. The pathogen is dispersed as a spore that can be carried for hundreds of miles under optimal weather conditions. The NC State Department of Plant Pathology maintains a downy mildew forcasting and tracking website that is very useful for growers. The website provides risk assessment for growing regions based on weather data and infection reports (insert link here).
After spores land on a susceptible host infection begins immediately. Symptoms can appear within three days although obvious signs of infection may not show for up to a week. Once symptoms appear, the pathogen is already producing more spores which can cause even higher levels of infection. Environmental conditions play a key role in how well the pathogen can infect and reproduce. Moisture is the most critical factor for downy mildew infection and this can be supplied by rain, dew or overhead irrigation. The ideal temperature for infection is 68º F although a range from 41º to 86º is sufficient. At temperatures above 86º, the pathogen can still survive and cause damage to the host but reproduction is limited.
Successful production of cucumbers in the spring or fall now requires the use of resistant cultivars and effective fungicides. See the North Carolina agricultural chemical handbook for effective fungicides. Many cultivars have been tested for downy mildew resistance in the field by the cucurbit breeding group at NC State University. The following tables summarize data from the 2004 to 2007 growing seasons.
Table 2. Leaf ratings for downy mildew damage on pickling cucumber cultivars for downy mildew in spring 2004 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
| Pickling cultivar |
Seed Source
|
Downy mildew rating
|
|
NC-Davie
|
NC State Univ.
|
2.3
|
|
Feisty
|
Harris Moran
|
2.7
|
|
Sassy
|
Harris Moran
|
3.3
|
|
NC-Leland
|
NC State Univ.
|
3.3
|
|
Johnston
|
NC State Univ.
|
4.0
|
|
M 21
|
NC State Univ.
|
4.0
|
|
Colt
|
Seminis
|
4.3
|
|
Vlasstar
|
Seminis
|
4.7
|
|
NC-Moriah (Beit Alpha)
|
NC State Univ.
|
4.7
|
|
Cross Country
|
Harris Moran
|
4.7
|
|
Jackson
|
Nunhems
|
5.7
|
|
Calypso
|
NC State Univ.
|
5.7
|
|
NC-Merritt (Beit Alpha)
|
NC State Univ.
|
5.7
|
|
NC-Lexington
|
NC State Univ.
|
5.7
|
|
Raleigh
|
NC State Univ.
|
6.0
|
|
NC-Duplin
|
NC State Univ.
|
6.0
|
|
Palomino
|
Seminis
|
6.3
|
|
NC-Longhurst
|
NC State Univ.
|
6.3
|
|
Wis. SMR 18
|
Univ. Wis.
|
7.3
|
|
LSD (5%)
|
2.4
|
Table 3. Leaf ratings for downy mildew damage on pickling cucumber cultivars for downy mildew in summer 2004 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
| Pickling cultivar | Seed source |
Downy mildew rating
|
|
Sassy
|
Harris Moran
|
5.0
|
|
Johnston
|
NC State Univ.
|
5.3
|
|
NC-Duplin
|
NC State Univ.
|
5.7
|
|
Jackson
|
Nunhems
|
5.7
|
|
M 21
|
NC State Univ.
|
5.7
|
|
Feisty
|
Harris Moran
|
6.0
|
|
Raleigh
|
NC State Univ.
|
6.0
|
|
NC-Davie
|
NC State Univ.
|
6.0
|
|
Vlasstar
|
Seminis
|
6.3
|
|
Calypso
|
NC State Univ.
|
6.3
|
|
Cross Country
|
Harris Moran
|
7.0
|
|
NC-Leland
|
NC State Univ.
|
7.0
|
|
NC-Lexington
|
NC State Univ.
|
7.0
|
|
Palomino
|
Seminis
|
7.3
|
|
Wis.SMR 18
|
Univ. Wis.
|
7.3
|
|
NC-Longhurst
|
NC State Univ.
|
7.7
|
|
Colt
|
Seminis
|
8.0
|
|
LSD (5%)
|
1.2
|
Table 4. Leaf ratings for downy mildew damage on slicing cucumber cultivars for downy mildew in spring 2004 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
| Slicing cultivar | Seed source |
Downy mildew rating
|
|
Poinsett 76
|
Cornell Univ.
|
2.0
|
|
Stonewall
|
Harris Moran
|
2.7
|
|
NC-Moriah
|
NC State Univ.
|
4.7
|
|
General Lee
|
Harris Moran
|
4.7
|
|
NC-Stratford
|
NC State Univ.
|
4.7
|
|
Cherokee 87
|
Check
|
4.7
|
|
Thunder
|
Seminis
|
5.0
|
|
Marketmore 76
|
Check
|
5.0
|
|
NC-Sunshine
|
NC State Univ.
|
5.3
|
|
NC-Merritt
|
NC State Univ.
|
5.7
|
|
Dasher II
|
Seminis
|
5.7
|
|
Speedway
|
Seminis
|
5.7
|
|
Ashley
|
Check
|
5.7
|
|
Intimidator
|
Seminis
|
6.0
|
|
Panther
|
Nunhems
|
6.0
|
|
Marketmore 86
|
Check
|
6.0
|
|
Revenue
|
Harris Moran
|
6.3
|
|
Coolgreen
|
Seminis
|
7.3
|
|
LSD (5%)
|
2.7
|
Table 5. Leaf ratings for downy mildew damage on slicing cucumber cultivars for downy mildew in summer 2004 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
|
Slicing cultivar
|
Seed source
|
Downy mildew rating
|
|
Dasher II
|
Seminis
|
4.7
|
|
Poinsett 76
|
Cornell Univ.
|
5.0
|
|
NC-Moriah
|
NC State Univ.
|
5.3
|
|
NC-Merritt
|
NC State Univ.
|
5.3
|
|
Thunder
|
Seminis
|
5.3
|
|
Ashley
|
Check
|
5.3
|
|
Speedway
|
Seminis
|
5.7
|
|
NC-Stratford
|
NC State Univ.
|
5.7
|
|
Intimidator
|
Seminis
|
6.0
|
|
Panther
|
Nunhems
|
6.0
|
|
NC-Sunshine
|
NC State Univ.
|
6.3
|
|
Stonewall
|
Harris Moran
|
7.0
|
|
Coolgreen
|
Seminis
|
9.0
|
|
LSD (5%)
|
1.4
|
Table 6. Leaf ratings for downy mildew damage on pickling cucumber cultivars for downy mildew in 2005 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
| Pickling cultivar | Seed source |
Downy mildew rating
|
| Fancipak | Seminis |
3.3
|
| Journey | Seminis |
3.7
|
| NC-Moriah | NC State Univ. |
3.7
|
| NC-Merritt | NC State Univ. |
3.7
|
| Pershing | Nunhems |
3.7
|
| Advance | Bejo Seed |
3.7
|
| Ballerina | Nunhems |
4.3
|
| Calypso | NC State Univ. |
4.3
|
| NC-Danbury | NC State Univ. |
4.3
|
| NC-Lexington | NC State Univ. |
4.3
|
| NC-Denton | NC State Univ. |
4.7
|
| Raleigh | NC State Univ. |
5.0
|
| Expedition | Seminis |
5.0
|
| Vlaspik | Seminis |
5.0
|
| NC-Dawson | NC State Univ. |
5.3
|
| NC-Dixon | NC State Univ. |
5.3
|
| Johnston | NC State Univ. |
5.7
|
| NC-Davie | NC State Univ. |
5.7
|
| NC-Duplin | NC State Univ. |
5.7
|
| Sumter | Clemson Univ. |
5.7
|
| Crispina | Nunhems |
6.3
|
| Wis. SMR 18 | Univ. Wisconsin |
6.3
|
| Coolgreen | Asgrow Seed |
6.7
|
| NC-Longhurst | NC State Univ. |
7.0
|
| Picklebush | Burpee Seed |
7.0
|
| LSD (5%) |
1.8
|
Table 7. Leaf ratings for downy mildew damage on slicing cucumber cultivars for downy mildew in 2005 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
| Slicing cultivar | Seed source |
Downy mildew rating
|
| Panther | Nunhems |
3.7
|
| Dasher II | Seminis |
4.3
|
| Marketmore 76 | Cornell Univ. |
4.3
|
| Stonewall | Harris Moran |
4.7
|
| Ashley | Clemson Univ. |
4.7
|
| Intimidator | Seminis |
5.0
|
| Poinsett 76 | Cornell Univ. |
5.0
|
| NC-Sunshine | NC State Univ. |
5.3
|
| General Lee | Harris Moran |
5.3
|
| Cherokee | Clemson Univ. |
5.7
|
| Talladega | Seminis |
6.0
|
| NC-Stratford | NC State Univ. |
6.0
|
| Spacemaster 80 | Cornell Univ. |
6.7
|
| Bush Champion | Burpee |
7.0
|
| LSD (5%) |
1.5
|
Table 8. Leaf ratings for downy mildew damage on pickling cucumber cultivars for downy mildew in 2006 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
| Pickling cultivar | Seed source |
Downy mildew rating
|
|
Cates
|
Nunhems
|
2.7
|
|
Wainwright
|
Nunhems
|
3.3
|
|
NC-Dawson
|
NC State Univ.
|
3.7
|
|
Rocket
|
Baker Seeds
|
4.0
|
|
Feisty
|
Harris Moran
|
4.0
|
|
Calypso
|
NC State Univ.
|
4.3
|
|
Ballerina
|
Nunhems
|
4.7
|
|
Vlaspik
|
Seminis
|
4.7
|
|
Powerpak
|
Seminis
|
4.7
|
|
Pershing
|
Nunhems
|
4.7
|
|
Classy
|
Harris Moran
|
4.7
|
|
Crispina
|
Nunhems
|
5.0
|
|
Starex
|
Baker Seeds
|
5.3
|
|
Raleigh
|
NC State Univ.
|
5.3
|
|
NC-Davie
|
NC State Univ.
|
5.3
|
|
NC-Denton
|
NC State Univ.
|
5.3
|
|
Vlasstar
|
Seminis
|
5.7
|
|
Atlantis
|
Bejo Seeds
|
5.7
|
|
Johnston
|
NC State Univ.
|
6.0
|
|
NC-Danbury
|
NC State Univ.
|
6.0
|
|
NC-Dixon
|
NC State Univ.
|
6.0
|
|
NC-Lexington
|
NC State Univ.
|
6.3
|
|
NC-Leland
|
NC State Univ.
|
6.3
|
|
NC-Duplin
|
NC State Univ.
|
6.7
|
|
Sumter
|
Clemson Univ.
|
7.0
|
|
NC-Longhurst
|
NC State Univ.
|
7.0
|
|
Wis. SMR 18
|
Univ. Wis.
|
7.3
|
|
H-19
|
Seminis
|
8.0
|
|
LSD (5%)
|
1.4
|
Table 9. Leaf ratings for downy mildew damage on slicing cucumber cultivars for downy mildew in 2006 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
|
Slicing cultivar
|
Seed source
|
Downy mildew rating
|
|
NC-Moriah
|
NC State Univ.
|
4.0
|
|
Thunder
|
Seminis
|
5.0
|
|
NC-Stratford
|
NC State Univ.
|
5.0
|
|
Poinsett 76
|
Cornell Univ.
|
5.0
|
|
NC-Merritt
|
NC State Univ.
|
5.3
|
|
Stonewall
|
Harris Moran
|
5.3
|
|
Impact
|
Western Seed
|
5.7
|
|
Talladega
|
Seminis
|
5.7
|
|
Intimidator
|
Seminis
|
6.0
|
|
Dasher II
|
Seminis
|
6.0
|
|
Ashley
|
Check
|
6.0
|
|
Speedway
|
Seminis
|
6.7
|
|
Panther
|
Nunhems
|
6.7
|
|
Cherokee 87
|
Check
|
6.7
|
|
NC-Sunshine
|
NC State Univ.
|
6.7
|
|
General Lee
|
Harris Moran
|
7.0
|
|
Marketmore 76
|
Check
|
7.3
|
|
Coolgreen
|
Seminis
|
7.3
|
|
LSD (5%)
|
1.6
|
Table 10. Leaf ratings for downy mildew damage on pickling cucumber cultivars for downy mildew in 2007 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
| Pickling cultivar | Seed source |
Downy mildew rating
|
|
NC-Davie
|
NC State Univ.
|
3.1
|
|
Cates
|
Nunhems
|
3.2
|
|
Picklet
|
Seminis
|
3.2
|
|
Eureka
|
Seminis
|
3.3
|
| Pony | Seminis |
3.3
|
|
Vlasstar
|
Seminis
|
3.5
|
|
NC-Duplin
|
NC State Univ.
|
3.6
|
|
Excel
|
Seminis
|
3.6
|
|
Fancipak
|
Seminis
|
3.6
|
|
Cross Country
|
Harris Moran
|
3.7
|
| Navigator | Seminis |
3.7
|
| Spunky | Harris Moran |
3.7
|
|
Wainwright
|
Nunhems
|
3.7
|
| Wellington | Seminis |
3.7
|
|
Classy
|
Harris Moran
|
3.8
|
|
Jackson
|
Nunhems
|
3.8
|
|
Journey
|
Seminis
|
3.8
|
|
Lafayette
|
Nunhems
|
3.8
|
|
Powerpak
|
Seminis
|
3.8
|
|
MacArthur
|
Nunhems
|
3.8
|
| Colt | Seminis |
3.9
|
|
Expedition
|
Seminis
|
3.9
|
|
Feisty
|
Harris Moran
|
3.9
|
|
Pershing
|
Nunhems
|
3.9
|
|
Starex
|
Baker Seeds
|
3.9
|
|
Vlaspik
|
Seminis
|
3.9
|
|
Arabian
|
Seminis
|
4.0
|
| Moxie | Harris Moran |
4.1
|
|
Sumter
|
Clemson Univ.
|
4.1
|
| Vlasspear | Seminis |
4.2
|
|
Calypso
|
NC State Univ.
|
4.3
|
| Vlasset | Seminis |
4.3
|
| Sassy | Harris Moran |
4.4
|
| Papillon | Seminis |
4.5
|
|
Greensleaves
|
Harris Moran
|
4.6
|
|
Atlantis
|
Bejo Seeds
|
4.8
|
| Palomino | Seminis |
5.2
|
|
Wis. SMR 18
|
Univ. Wis.
|
5.9
|
| National Pickling | NSSL |
6.3
|
|
LSD (5%)
|
0.5
|
Table 11. Leaf ratings for downy mildew damage on slicing cucumber cultivars for downy mildew in 2007 (leaves rated 0 to 9; 0=no disease, 1-2=trace, 3-4=slight damage, 5-6=moderate damage, 7-8=severe damage, 9=dead).
|
Slicing cultivar
|
Seed source
|
Downy mildew rating
|
|
Poinsett 76
|
Cornell Univ.
|
3.7
|
|
Stonewall
|
Harris Moran
|
3.7
|
|
Impact
|
Western Seed
|
3.8
|
| Diamante | Harris Moran |
3.9
|
|
Slice
|
Clemson Univ.
|
3.9
|
| Fanfare | Seminis |
4.2
|
|
Indy
|
Seminis |
4.2
|
|
NC-Stratford
|
NC State Univ.
|
4.2
|
| Tablegreen 72 | Cornell Univ. |
4.4
|
|
General Lee
|
Harris Moran
|
4.5
|
|
Panther
|
Nunhems
|
4.5
|
|
Dasher II
|
Seminis
|
4.6
|
|
Intimidator
|
Seminis
|
4.6
|
|
Marketmore 76
|
Check
|
4.6
|
|
Thunder
|
Seminis
|
4.7
|
|
Ashley
|
Check
|
4.8
|
|
NC-Sunshine
|
NC State Univ.
|
5.0
|
|
Talladega
|
Seminis
|
5.1
|
|
Straight 8
|
NSSL
|
6.3
|
|
Coolgreen
|
Seminis
|
6.5
|
|
LSD (5%)
|
0.5
|
The main difference for downy mildew control for cucumbers grown in North Carolina after 2004 is that resistant cultivars must be grown, AND effective fungicides must be applied. See the North Carolina agricultural chemical handbook for effective fungicides.