Cucurbit Downy Mildew in Western NC

Cucurbit downy mildew has been identified on cucumbers in Buncombe and Haywood Counties.  Growers are urged to scout their fields and begin preventive management.

Review control measures and previous posts on this major disease issue - Cucurbit downy mildew

Water soaked lesions, caused by Pseudoperonospora cubensis (causal agent of cucurbit downy mildew), visible on the underside of cucumber leaf.

Also, please visit the Cucurbit Downy Mildew Forecasting site to see more pictures of the disease, review control measures, report disease outbreaks in your area and more.

Downy Mildew Confirmed in Buncombe County

Downy mildew has been confirmed at very low levels in a 2-acre cucumber field in Buncombe County. Growers are encouraged to begin treatment.

Review this post from June on the current recommendations: Pest News June 24

Don't forget to review the Cucurbit Downy Mildew Forecasting website for more information.

Pest News June 24

The latest edition of the NC Pest News is now available.

In this latest edition:

1. Cucurbit downy mildew recommendations for 2011
2. Cucurbit powdery mildew recommendations fro 2011

Cucurbit downy mildew has been found in NC in more eastern counties. It has not been found in WNC, yet. We are actively scouting as afternoon thundershowers and storms from the south could provide inoculum and suitable conditions for disease development. There have been no forecasts on the Cucurbit Downy Mildew Forecasting Website for disease development in our area. However, if you think you have this disease, please visit your local extension office for confirmation.

Downy mildew symptoms on cucumber leaf.

Downy mildew sporulation on the underside of a cucumber leaf.

The fungus that causes cucurbit powdery mildew is a very different pathogen than the water mold pathogen that causes cucurbit downy mildew. Cucurbit powdery mildew likes warm, humid conditions (the humidity within a plant canopy provides ideal conditions). The pathogen does not like wet conditions, but water will spread the pathogen spores to other plants.

Powdery mildew on a squash leaf.

Powdery mildew on the stem of a squash plant.

Downy Mildew on Cucumber in WNC

Well, this has been a very exciting week to be a plant pathologist! First, downy mildew on basil, now downy mildew on cucumbers.
*These are two different disease problems, just because they have the same name DOES NOT mean that these two disease are caused by the same organism and will cross-infect one another!*

Cucurbit downy mildew, caused by pathogen Pseudoperonospora cubensis, has been confirmed on pickling cucumbers in Henderson County. Growers are advised to scout their cucurbit plantings which include cucumber, squash, melons and other gourds. Last year we found the downy mildew in Henderson County late in July.

Symptomatic downy mildew cucumber leaf. Angular lesion are first light green and then turn brown with age.

The underside of a downy mildew infected cucumber leaf. You can see the dark spores of the pathogen as they arise from the angular lesions which are bound by the leaf veins.

Downy mildew of cucurbits has the potential of spreading very quickly. Its wind-borne spores can spread easily throughout the region - especially on cloudy days. The pathogen prefers cool temperatures and wet conditions, which we certainly have not had in WNC as of late! I believe that recent hot, dry and sunny conditions are the reason that the downy mildew on cucurbits is not widespread at this time. For example, large cucumber plantings just miles away from where I initially found the disease are unaffected at this time.

With rain possible this weekend, it is very important to prevent downy mildew from infecting your cucurbits, especially cucumber. Growers are advised to protect their plants using fungicides. For conventional producers, review the June 4th North Carolina Pest News or review my blogpost from June 2009 for control recommendations.

Management of downy mildew on cucurbits is tricky for organic producers. There are a number of OMRI-listed products that are labeled for the control of downy mildew on cucurbits, including copper, OxiDate, Serenade and neem. In research studies, copper products were proven to be the best option (only option, really) for managing downy mildew on cucurbits. Copper is only efficacious in plants that are not yet infected or exhibit only mild symptoms.

Visit the Cucurbit Downy Mildew Forecasting Website to see where the disease is currently located and where the team predicts it to show up next.

If you need help diagnosing a problem on your cucurbits, please contact your local NC Cooperative Extension Office.

To see more pictures of the downy mildew on cucurbits and to learn more about the disease, visit my previous posts.

Basil Downy Mildew

Sporulation of Peronospora behlbarii, causal agent of basil downy mildew.

Basil downy mildew is a new disease that has been causing headaches for growers in the eastern US for the past two seasons. Basil downy mildew was reported in South Florida in October 2007. This disease has the potential to be very destructive to basil producers. European producers have been dealing with the disease since 2001. The disease is thought to have been introduced in the US from infected seed from Europe

The first confirmed case of the disease in NC occurred last summer in Chatham County. Agriculture agent, Debbie Roos, did a great post on the diseases with wonderful pictures last August.

The disease has been found in several locations in WNC this year. This is surprising because we have been so hot and dry and downy mildew pathogens tend to like cooler temperatures and wet and humid conditions. Apparently, basil downy mildew is the exception - it likes moderate to warm temperatures!

Basil downy mildew is so devastating because the discoloration of the leaves makes the basil unmarketable. Eventually the leaves will become black or necrotic and die completely.

Slight yellowing of basil plants that are infected with Peronospora behlbarii, causal agent of basil downy mildew.

Discoloration of basil leaf caused by downy mildew.

The basil downy mildew pathogen, Peronospora belbahrii, is spread by contaminated seed, marketing infected plants or leaves and wind-borne spores. This pathogen also infects other plants of the Lamiacea family which inculdes basil, sages, and mints. The spores of the pathogen are abundant on the underside of the leaf.

Discoloration of basil leaf caused by downy mildew, P. behlbarii.

Notice the fuzzy growth on the underside of the leaf. This was the first tip that what we were dealing with was downy mildew. Also, note the black dying tissue.

Dark sporulation of the pathogen P. behlbarii that causes downy mildew on basil on the underside of the leaf.

This disease can often be mistaken for sunburn on basil leaves, which I have also noted this year.

Sunburned basil leaves.

There is a similar downy mildew pathogen that infects coleus, but this pathogen has been shown to be genetically different than the pathogen on basil and the ornamental hosts are no longer considered alternative hosts to the basil disease. To read more about the specifics of basil downy mildew, visit Cornell Plant Pathologist Dr. Meg McGrath's on-line factsheet Basil Downy Mildew - A New Disease to Prepare For.

Management of basil downy mildew requires an integrated approach. Using seed that is not contaminated with the pathogen, selecting less susceptible varieties and applying fungicides are the primary practices. Management should also include increasing airflow through the plant canopy in order to minimize leaf wetness and humidity in order to suppress the disease. This is especially true in greenhouses. Dr. McGrath outlines management steps, including variety trials from New Jersey, in her factsheet.

Fungicides used for downy mildew on basil include OMRI-approved Actinovate and OxiDate, which are labeled for use. Many other conventional fungicides do not yet have the label required to control this disease on basil, with the exception of K-Phite and ProPhyte. It is expected that this will change in the near future. Fungicides should be applied frequently and prior to first symptoms if possible in order to control downy mildew on basil effectively. This is of course tricky, especially in areas without history of the disease.

If you suspect that you have downy mildew on your basil, visit your local NC Cooperative Extension office for help identifying and reporting the disease.

Downy Mildew on Cucumber Confirmed in Eastern NC!

Downy mildew has been confirmed on cucumbers in Columbus County (May 27) and Duplin County (June 1). Both of these counties are located in eastern NC.

Downy mildew has not been found on cucurbits in WNC yet this season and current forecasts do not indicate risk to our production. If the forecasts show risk for our area, or downy mildew is found in our region, I will update the blog.

The Cucurbit Downy Mildew Forecasts can be found at http://cdm.ipmpipe.org/.

Current recommendations for cucurbit downy mildew can be found in the latest issue of North Carolina Pest News.

Recommendations for powdery mildew on cucurbits can be found in this issue, as well as information on late blight of tomatoes.

Downy Mildew on Hops Present in WNC

This week I have been visiting with our WNC hops growers. To my dismay, I found the dreaded downy mildew, Pseduoperonospora humuli, in two of the yards.

We saw the beginnings of this problem late last season on the hop cones. See my post from September 2008 on downy mildew of hops.

When determining if you have downy mildew in your hops yard, look for pale green to yellow newly emerged spikes that appear stunted.

Note the pale green, stunted spikes of this hops plant.

A closer look at the pale green to yellow new spike. Note the shorten internodes and miniature leaves.

Another shot of the infected, newly emerged shoot. Take note of the curled and brown lower leaves.

A close up of the browning, curled leaves on the hops shoot. The leaf looks dirty, but those are actually millions of downy mildew sporangia!

A hops leaf exhibiting foliar symptoms of downy mildew.

The underside of the downy mildew infected leaf. Note the fuzzy spores.

The worst downy mildew infection that I noted was on 'Nugget'. 'Nugget' is considered to be susceptible to downy mildew. Interestingly, in the hops yard where downy mildew was found on 'Nugget', it appeared to be the most vigorous variety.
**Update May 21**
Some sources report that 'Nugget' is resistant to downy mildew, while others report it is susceptible.

Downy mildew was also found to be prevalent on 'Centennial' which is considered to be tolerant to downy mildew.

One of the best management strategies for the control of downy mildew on hops is to remove primary basal spikes and to heavily prune and strip leaves. This is important in order to prevent downy mildew from moving up the bines and eventually infecting the cones. Pruning and thinning also helps to reduce moisture and humidity in the lower part of the canopy which is favorable for disease development.

Hops yard with lower leaves stripped up to 3 feet.

Because we are expecting wet weather in the next few days here in WNC, action needs to be taken quickly to prevent the spread of disease to other bines. The first step is to prune and thin the bines.

In addition, the application of fungicides is recommended as soon as the wet weather is over or before if product is rainfast. For a complete list of fungicides (conventional and organic) used to control downy mildew on hops, visit the Oregon State University Downy Mildew Fact Sheet. Before you apply any pesticide make sure you follow the specific label directions and make sure that the product is labeled for downy mildew on hops in NC.

*UPDATE*

Organic Fungicides Labeled for Management of Downy Mildew of Hops in NC:

• Sporatec (rosemary oil, clove oil and thyme oil)
• Copper products (look for OMRI label)
• Sonata (Bacillus pumulis strain QST 2808)
  

Conventional Fungicides Labeled for Management of Downy Mildew of Hops in NC (not a complete list):

• Curzate (cymoxanil) - must be tank mixed with a fungicide of different mode of action, such as copper
  
• Tanos (cymoxanil + famoxadone) - must be tank mixed with a fungicide of different mode of action, such as copper
• Acrobat and Forum (dimethomorph) - must be tank mixed with a fungicide of different mode of action
• Fosphite and other phosphorous acid products
• Legion (Aluminum tris)
  

The best approach would be to incorporate a few of these products in order to slow down the possibility of fungicide resistance. Again, always follow the directions on the label - it's the law!

Downy Mildew Confirmed in Henderson County!

Downy mildew has been confirmed at 2 locations in Henderson County, NC.

• July 20 - The first incidence of cucurbit downy mildew in WNC was reported from the Mountain Horticultural Crops Research Station in Mills River. The hosts were butternut squash and cucumber. Disease incidence and severity is very low at that site.

• July 24 - Cucurbit downy mildew was confirmed this morning in Zirconia (just south of Hendersonville, NC). This was a larger planting, about 8 acres. Spaghetti squash is the most severely diseased, about 65% of plants are showing early (top picture) - more advanced (second picture) symptoms. Sporulation (noticeable on the underside of the leaf) was very heavy on the spaghetti squash (third picture). Cucumber (cv. Jackson) is also affected, though less severely overall (bottom picture). The summer squash (cv. Destiny III) is showing only very early and mild symptoms.

Moral of the story, cucurbit downy mildew is in WNC and growers need to start treating now!

Review a previous post for control recommendations.

Visit the Cucurbit Downy Mildew Forecasting site for more updates.

If you feel you have downy mildew on your cucurbits, bring a sample to your local county cooperative extension office for positive identification. It is important that the outbreaks get reported to the forecasting site.

Downy Mildew Confirmed on Cucumber in Franklin County, NC

We have had our first report of the year of downy mildew on cucumber in NC. The outbreak is in Franklin Co, NC, located in the northeast of Wake County (Raleigh).

This disease favors the cool, wet weather we have been experiencing here in NC. Though we have not had any confirmed cases in WNC, it would be worth your time to scout your fields. Also, follow the Cucurbit Downy Mildew Forecasts for updated information.

Downy mildew of cucurbits is a disease that is very difficult to control once it becomes established in your field and can spread very quickly (see pictures below- Top picture = healthy cucumber field, Bottom picture = same field a few weeks later). Research has shown that by delaying fungicide sprays to cucumber just one week after the disease has become established in the field, yield losses of greater than 50% can occur (especially in cool, wet weather).



The disease spreads via wind currents from downy mildew infected areas. It can also be spread throughout a field by water splashes.

Symptoms on cucumber, squash and melon begin as yellow spots on the leaf. The spots are bound by the leaf veins and, as a result, appear angular. On the underside of the leaf, the spots may appear water-soaked, especially in early morning when moisture is still present in the plant canopy. You may also see a light gray to dark almost black, downy growth. Those are the spores (zoosporangia) of the pathogen, Pseudoperonospora cubensis. Eventually these lesions, and then the entire leaf, will become necrotic and turn brown. The leaf may appear crispy, like it has been burned. Because of rapid onset and crispy leaves, this disease has been referred to as "wild fire:


The following products* have proven very effective for the control of downy mildew:

Presidio 4F (fluopicolide)
Ranman 400 CS (cyazofamid)- must use with non-ionic surfactant
Gavel 75DF (zoxamide + mancozeb)
Curzate 60DF (cymoxanil)

Chlorothalonil and mancozeb products can be sprayed as protectants, but when downy mildew is detected in a field growers need to take a more aggressive approach. *The label on some of these products require that they be tank mixed with a fungicide of a different mode of action. Chlorothalonil and mancozeb products make excellent mixing partners.

A sample spray program might look like this:

Curzate 60DF (3.2 oz/acre) + Manzate Pro-Stick 75DG (3 lb/acre) alternated with Previcur Flex 6F (1.2 pt/acre) + Manzate Pro-Stick 75DG (3 lb/acre)

OR

Presidio 4F (3 fl oz/acre) + Bravo Weather Stik 6SC (2 pt/acre) alternated with Gavel 75DF (1.5 lb/acre)

These treatments can be rotated on a 7-day schedule when downy mildew pressure is great.

To read more about this disease visit the NCSU Factsheet.
In addition, look for updates in the latest NCSU Pest News.

***As always, follow the product label for instructions!***