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Cucumber Crop Information

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.

Downy mildew photo

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.


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