Over the last several years, the number of questions I have had about tomato production in high tunnels has increased dramatically. Since I am a plant pathologist, most of the questions I have been asked are about diseases of tomatoes in high tunnels. However, I also have been asked production questions. One particular question about tomato product that may impact disease severity is this: how many staked tomatoes can be grown in a high tunnel effectively?
To be honest, the above question is one that I often ask myself when I observe high tunnels in Indiana. It isn’t necessarily one that is asked by growers. But maybe it should be. It has been my observation that growers often try to place too many staked tomatoes in a high tunnel. The result may include diseased tomato plants due to insufficient air circulation, poor quality fruit and even reduced yields. I was not able to find any information about plant spacing of staked tomatoes in high tunnels. So, I decided to research the subject myself.
Let me take a moment to insert some definitions. A high tunnel is a greenhouse with passive heating. The only heat is that supplied is from sunshine trapped in the structure during the day. The structure is cooled by raising the sides and/or venting the top. Staked tomatoes are determinant tomatoes that grow only to a specific height. This is in contrast to indeterminate tomatoes that are trellised to the ceiling and back down again indefinitely. The study that I will describe here is for tomatoes that are grown in the ground as opposed to in pots are bags.
The 2014 staked tomato study
The 2014 study included two varieties at 5 different plant populations. Both varieties, Red Deuce and Mountain Spring are determinant. Each variety was grown at one of 5 different plant in-row spacing’s (plant population): 18 inches-Florida weave, 20 inches-Florida Weave, 20 inches Spanish trellis, 24 inches-Florida weave, 28 inch-Florida weave. In the Florida weave method, a wooden stake was placed every two plants and twine was woven around the plants and stakes to keep the plants upright. The Spanish trellis technique placed 4 stakes around two tomato plants with an X-pattern of twine woven between the stakes. In each case, twine was tied every 8 to 12 inches of plant growth. It is important to realize that although I varied the spacing of plants within a row, I did not vary the width between rows (center-to-center) which remained at 5 feet for all treatments.
The influence of varieties and plant populations on plant disease
It was my hypothesis that tomato plants grown at narrow spacing’s would have less ventilation, higher humidity and more disease when compared with tomato plants at more distant spacing’s. In the first week of August, the disease leaf mold was observed on the tomatoes in the experiment. I took observations of disease every few days.
While I observed leaf mold on the variety Mountain Spring, I never observed any leaf mold on Red Deuce. This was surprising since Red Deuce was not listed as resistant. It is always a good idea to choose a variety with host resistance, but also note the disease reaction of your plants regardless of how the variety is listed.
Regardless of what I thought would happen, I did not observe more disease on varieties at closer spacing’s. Does this mean that growers should space tomatoes very close together?
I think there are several factors which may have influenced whether the narrow spacing became more diseased.
- · If I could have planted the entire high tunnel to a narrow spacing, then ventilation/humidity may have been affected by in-row spacing differences.
- · Center-to-center spacing may be more important in disease management than in-row spacing. Remember that I did not vary center-to-center spacing.
- · We use a high tunnel that has a ridge vent. It is possible that such a high tunnel design reduces relative humidity over a design without ridge vents.
- · These results were for the disease leaf mold. Results with other diseases may vary.
Further, placing tomatoes very close together may have other repercussions—read on.
The influence of varieties and plant populations on weight and number of tomatoes
Growers might be interested in knowing how these treatments affected the yield in weight and number of fruit.
Variety-There was no significant difference in the yield in weight between Red Deuce and Mountain Spring when calculated over the entire season. However, there were significantly more Mountain Spring tomatoes than Red Deuce. (I noted earlier here that the Mountain Spring tomatoes had physiological leaf roll. Given the results of this study after one year, it appears that the leaf roll on the Mountain Spring did not hurt the yield.) Red Deuce yielded significantly larger fruit than Mountain Spring.
Plant populations-There was no difference in the weight of fruit harvested per linear foot when analyzed by plant population when calculated over the entire season. However, the number of fruit per linear foot was less on plants spaced at 24 and 28 inches compared to plants spaced at 20 inches on a Spanish trellis.
The influence of varieties and plant populations on the mean weight of tomatoes
Many growers are able to obtain a premium for larger tomatoes. So, I also looked at whether the variety and plant population influenced the size of tomatoes. When calculated over the entire season, Red Deuce had larger (heavier) fruit than Mountain Spring. Perhaps more important, plants at narrow spacing’s tended to have smaller fruit than plants at more distant spacing’s. For example, plants at 16 inches with Florida Weave and 20 inches with Spanish Trellis had significantly larger fruit than plants spaced at either 24 or 28 inches with Florida weave.
Take Home message
Plant disease-leaf mold disease, while severe on Mountain Spring tomatoes, did not affect Red Deuce tomatoes in this experiment. Plant populations did not influence the amount of leaf mold on tomato plants.
Yield-I was not able to observe any difference in yield in pounds for tomato fruit when I measured over the entire season. However, I did see more tomato fruit on plants spaced closer together. Mountain Spring had more tomatoes than Red Deuce.
Mean fruit weight-Growers who are interested in larger tomatoes will want to avoid the closest spacing’s tried here. Plus, Red Deuce had larger tomatoes than Mountain Spring.
My plan is to repeat this experiment in the summer of 2015. More detailed recommendations will be made when the results of both years of experiments have been analyzed. What follows, however, are my thoughts so far. The most effective between row spacing (center-to-center) should be about 5 feet. In-row spacing should be from 20 to 24 inches.
Watch this space for more information or updates.
Last week, I observed white mold of recently transplanted tomato plants in a greenhouse situation. I have described the symptoms, biology and management of white mold here.
I have never observed white mold (a.k.a, timber rot) in February before. I have observed white mold of tomato transplants in April. However, the very small mushroom (smaller than a dime) that is part of the life cycle usually emerges in the spring after a cold period.
The appearance of white mold in February may be as a result of the presence of the mushroom in the greenhouse that produced the transplants.
To reduce severity of white mold of tomato, I recommend that tomato growers:
Contans is a product labeled for greenhouse use. It must be worked into the soil months before the tomato crop is planted. See the Midwest Vegetable Production Guide for Commercial Growers, 2015 ( ID-56) or see the Contans label for more information.
· Inspect transplants for stem lesions which may be a symptom of white mold. Bring questionable symptoms to my attention or send them to the Purdue Plant and Pest Diagnostic Laboratory.
· Clean and sanitize greenhouses in-between tomato transplant generations.
· Use a floor covering to reduce the chance of crop residue getting in the soil. A floor covering should also reduce weeds and the white mold mushrooms.
White mold, also known as timber rot, on a tomato transplant. The stem of the seedling has broken at the point of the white mold lesion. Note the white fungus present on the lesion.
(Click to see larger image)
The Midwest Vegetable Production Guide for Commercial Growers 2015 (ID-56) is now available on-line and as a hard copy from Purdue University. Please read below for an update to the 2015 version of this guide.
What’s New in 2015?
Highlights of Changes in This Edition
New and Revised Sections
• Table 33: Common and Scientific Vegetable Pest Names (page 74) lists the common and scientific names of the pests listed in this guide.
• Table 18: Sanitizers Approved for Wash or Process Water (page 46) lists federally approved sanitizers for vegetables.
• Table 12: Yields of Vegetable Crops (page 37) has been updated.
• White rust has been added to the Cole Crops and Brassica Leafy Greens chapter.
• Bacterial fruit botch recommendations have been revised in the Cucurbit Crops chapter. • Merivon® has been added to the Cole Crops and Brassica Leafy Greens, Cucurbit Crops, and Dry Bulb and Green Bunching Onion, Garlic, and Leek chapters.
• Priaxor® has been added to the Fruiting Vegetables chapter.
• Aproach® has been added to the Legumes chapter.
• Command 3ME® is now labeled for use on broccoli, cauliflower, Brussels sprouts (see the Cole Crops chapter), and rhubarb.
• League ® is now labeled for use in pepper, tomato, potato, cantaloupe, and watermelon. The active ingredient is imazosulfuron, an ALS-inhibiting herbicide. This material has pre- and postemergent activity against yellow nutsedge, hairy galinsoga, purslane, and pigweeds, and postemergent activity against morningglory.
• Azera® was added to the Cole Crops and Brassica Leafy Greens and Cucurbit Crops chapters.
• Beleaf 50SG® was added to the Cole Crops and Brassica Leafy Greens, Cucurbit Crops, Fruiting Vegetables, Potato, and Sweet Potato chapters.
• Exirel® was added to the Cole Crops and Brassica Leafy Greens, Cucurbit Crops, Fruiting Vegetables, and Dry Bulb and Green Bunching Onion, Garlic, and Leek chapters.
• Torac® was added to the Cole Crops and Brassica Leafy Greens and Potato chapters.
• Transform® was added to the Legumes and Potato chapters.
• Zeal® was added to the Mint and Okra chapters.
Updates to the Midwest Vegetable Production Guide-These changes appear in the on-line version of the ID-56. Please make these changes to your hard copy.
· Bravo Weather Stik was removed from the cucurbit powdery mildew management section
· Merivon was added to the cucurbit powdery mildew management section.
Bacterial fruit blotch is a disease that can affect most cucurbits. However, the symptoms are most often observed on watermelon. A brief description of this disease and some photos can be found here. This article will introduce new recommendations for this disease in Indiana. Details of these recommendations can be found in the Midwest Vegetable Production Guide for Commercial Growers 2015 (ID-56). Hard copies of the ID-56 are available from Purdue University now for $10. A free on-line version of the ID-56 will be available soon at mwveguide.org.
Copper products such as those with copper hydroxide or copper sulfate are often recommended for management of bacterial fruit blotch (BFB). However copper products applied too often can cause yellowing of leaves and even yield loss (phytotoxicity). Since BFB is mostly caused by rare contaminated seed lots, I have been reluctant to recommend copper products routinely for watermelon growers. However, the last few years I have observed several growers suffer large losses from this disease. Therefore, I have decided to bring Indiana recommendations in line with those of Georgia and South Carolina. See below.
Instead of applying copper products all season long, apply copper 2 weeks before first female bloom, at first female bloom and 2 weeks after first female bloom. Additionally, application of the product Actigard at 2 of the 3 copper application times listed above is recommended.
Actigard does not have any direct effect on the bacterium that causes BFB. Instead, Actigard ‘tells’ the plant that it is under attack from a disease, causing the plant to produce defense compounds. Like copper compounds, Actigard can cause yield losses in plants. Therefore, do not use Actigard on plants that have been stressed by weather or other factors. I would use the low rate of Actigard, 0.5 oz. per acre.
If you have any questions or comments about bacterial fruit blotch, these new recommendations or the Midwest Vegetable Production Guide for Commercial Growers 2015 (ID-56), please do not hesitate to contact me.
While visiting my son in Lincoln, Nebraska this past summer, I had the chance to browse in a second hand store. I felt myself drawn to the book section where I found a green hard cover book titled, “Yearbook of Agriculture, 1928”.
From 1894 until 1992, the Department of Agriculture published a Yearbook of Agriculture annually. These books provided updates, features and statistics for the year. The reports actually go all the way back to 1862, when the head of the agriculture department, Isaac Newton, submitted a report to the Commissioner of Patents. (It turns out most of these books have been scanned and are on-line-I could have saved myself $1.50 had I known!)
It is my plan to report on parts of the 1928 book that I think might interest vegetable growers in Indiana. The first subject which caught my eye were the statistics for processing tomato production.
Below I have constructed a table of comparison of processing tomato production in 1928 and in 2013, the latest year for which there are statistics. To be honest, the table from which the 1928 data comes from is titled, ‘Tomatoes for manufacture….”. I assume that is the same as the 2013 category of “Tomatoes for Processing” in the Vegetables 2013 Summary, USDA, National Agricultural Statistics Service.
The first thing I noticed was the decline in acres of processing tomato production between 1928 and 2013-a factor of more than 5! But, don’t despair…..although acres of tomatoes went down, production went way up. The reason is obvious-yield per acre of processing tomatoes went up 10X. I suspect tomato varieties in 1928 were open pollenated vs. the hybrids of today. The use of transplants for tomato production probably started after 1928. No doubt there are other reasons for this success story.
Prices for a ton of processing tomatoes have gone up considerably as well. For comparison, field corn prices went from $0.82 in 1928 to $6.22 in 2013 (Iowa State Extension figures). Corn prices increased a factor of about 7X, vs the almost 10X price increase for processing tomatoes.
One more item not shown in this table: the ranking of the tomato processing industry in Indiana nationally. California today leads the nation in processing tomato production with Indiana second. In 1928, California was a distant second to Indiana in acres of processing tomatoes with only 25,790 acres. However, California produced 201,200 tons, out producing Indiana (see below). It may be that in 1928, California had advanced technology that Indiana had not yet adopted. Today, Indiana’s processing tomato industry is well versed in the best methods of production. California’s lead in tomato production today is primarily in total acres.
As I find more interesting tidbits of information about agriculture in 1928, I will discuss those concepts here.
Comparison of processing tomato production in Indiana in 1928 and in 2013.
Yield Per acre (Tons)
*Source: Yearbook of Agriculture, 1928.
**Source: Vegetable 2013 Summary, USDA, National Agricultural Statistics service.
For 100 years bacterial spot has been causing huge losses for tomato growers worldwide. For 100 years products containing copper have held the key to controlling this devastating tomato disease. As tomato growers enter their second century of dealing with bacterial spot, the question has become whether copper applications lessen the severity of bacterial spot-or perhaps even make the disease worse. This article will discuss bacterial spot of tomato, why copper products have become less useful in the control of this important disease and finish with options for managing bacterial spot of tomato with and without copper.
The first symptoms of bacterial spot one is likely to observe are small, less than 1/8 inch dark lesions on tomato leaves. The lesions may appear watersoaked, especially in the morning and are often surrounded by yellow (chlorotic) tissue. These lesions, whether found on leaves or stems, may coalesce to cause the loss of large areas of plant tissue. Loss of yield or fruit quality may result from foliage lesions or lesions that occur on fruit. For a link to the Purdue Tomato Doctor app, click here.
Today, copper products are used primarily to manage bacterial diseases. At one time, however, copper was perhaps the most important fungicide as well. The use of copper to control diseases of plants goes back to 1882 when Professor Millardet noticed that the use of a copper substance on grape leaves to deter thieves also protected the vines from downy mildew. While initially crude copper substances were applied to leaves, copper products were refined so that small particles of insoluble copper compounds could be applied, remaining on the leaves for a significant period of time. Such products became known as ‘fixed copper’ products.
Heavy metal elements, such as copper, were often the products of choice to manage plant diseases until the 1930’s and 1940’s when synthetic fungicides began to be produced. Since that time, it has been recognized that, in general, synthetic compounds are more effective against fungal diseases than elemental based products such as copper hydroxide. Bacterial spot of tomato is one of the diseases for which copper products has remained an important management tool.
The repeated application of copper products, however, did not always control bacterial spot of tomato. Scientists began to turn up evidence that the bacteria that cause diseases such as bacterial spot of tomato became resistant to copper over time. It seemed that the more copper applications that were made, the faster the bacteria became resistant to copper.
Since bacteria first evolved on earth billions of years ago, they have had to find ways to get rid of heavy metals like copper. When humans began to use copper to try to control bacterial diseases, genetic traits that allowed bacteria to overcome copper toxicity have become more frequent in the bacterial population. In 1986, Robert Stall of the University of Florida found that the trait that allows strains of bacterial spot pathogen to resist copper could be found on a small piece of DNA (known as a plasmid) that is easily transferred to other bacteria. Therefore, this important piece of DNA that codes for copper resistance could be easily spread within bacterial populations.
Although there was increasing evidence of copper resistance in strains of bacterial spot of tomato, copper products continued to be used as an important tool. Some of the techniques used to increase control with copper included increasing the application frequency of copper products, increasing the amount of copper applied and mixing copper with the product mancozeb to increase the amount of copper available on the leaf surface.
The fact that many strains of the bacterial spot pathogen were resistant to copper was bad news. Recently, however, even worse news was reported. The new evidence shows that, in at least some cases, the use of copper actually makes bacterial spot worse than not using any copper at all.
Why would the use of copper result in more bacterial spot of tomato? No one knows for sure. It is probably safe to assume that in such circumstances, almost all the bacteria present are resistant to copper. The use of copper therefore did not lessen the disease. Plus, it may be that the copper product eliminated all the beneficial micro flora (bacteria and fungi) on the leaf, making the leaf an excellent environment for bacterial spot.
At a recent Tomato Disease Workshop in Windsor Canada, whether or not to recommend copper products for bacterial spot was one of the topics discussed. Most extension workers will leave copper products in the list of recommendations for bacterial spot of tomato. The reasons are:
1. 1. One of the most prominent studies that show copper products may make bacterial spot of tomato more severe were conducted at the University of Florida (by Professor Gary Vallad). Weather conditions are near perfect for bacterial spot in Florida, perhaps better than anywhere else in the U.S. It is possible that with less conducive weather conditions (like in Indiana), copper products may be able to perform better.
2. 2. Because bacterial spot is so much more severe in Florida than anywhere else, more copper applications are used in Florida, leading to higher proportions of copper resistance in bacterial populations. In Indiana, for example, it is probable that the percentage of copper resistant bacteria is lower than in Florida. Observations suggest that copper products are still useful in Indiana.
3. 3. There are very few good answers to managing bacterial spot of tomato. While copper may not be the answer it once was, the lack of good answers makes this extension worker hesitate to cross copper products off the list. Indiana tomato growers will find that copper products are still one of the options listed in the Midwest Vegetable Production Guide for Commercial Growers, 2015 (expected in January).
Since copper products alone will not control bacterial spot of tomato, what other options do we have?
Although it has been said many times, many ways, crop rotation is an excellent method to lessen the severity of bacterial spot. Or, to put it another way, if you don’t rotate, bacterial spot will almost surely become a big problem.
Although there are no tomato varieties that are resistant to bacterial spot, varieties vary in susceptibility. Ask your seed company representative about bacterial spot and host susceptibility. Make your own observations about bacterial spot susceptibility in your field on your varieties.
Always choose tomato seed that have been tested for the bacterial spot pathogen. Consider treating seed for bacterial spot (see the MidwestVegetable Production Guide for Commercial Growers, 2015). Avoid, if at all possible, growing transplants of different varieties together in the same greenhouse. Scout transplant greenhouses carefully for symptoms of bacterial spot. Do not plant transplants from a greenhouse where bacterial spot has been found. Carefully clean and sanitize transplant greenhouses in between generations of transplants.
Water is necessary for bacterial spot of tomato to initiate disease and for disease spread. Any practice that lowers the amount of leaf moisture will lower the amount of disease. For example, overhead irrigation should be avoided in favor of drip tape. If overhead irrigation is unavoidable, do not irrigate in the evening when the tomato leaves will likely remain wet through the morning dew period.
Tomatoes grown to maturity in greenhouses or high tunnels often do not have severe symptoms of bacterial spot of tomato. The greenhouse covering will, for the most part, keep moisture from forming on leaves and prevent rains from splashing bacteria from leaf to leaf. Thus, growing tomatoes in greenhouses or high tunnels is one method to lessen the severity of bacterial spot. (However, diseases such as leaf mold, gray mold and white mold are often more prominent in a greenhouse than in a field.)
Products with streptomycin (e.g., Agri-mycin 17®, Firewall®, Harbour®) can be used in the transplant greenhouse (Streptomycin products cannot be used on field tomatoes). The use of streptomycin products will help to lower the populations of strains that cause bacterial spot, including those strains that are resistant to copper. It makes sense to use streptomycin products at least once and a copper product at least once in a tomato transplant house. If both applications are effective, the only strains remaining will be strains that are resistant to both copper and streptomycin-a small sub population hopefully.
Products with the active ingredient hydrogen dioxide (e.g., Oxidate®) are also labeled for bacterial spot in the greenhouse. Hydrogen dioxide can kill bacteria on contact, however, it has very little to no residual. That is, an hour or so after application, there will be no activity from these products. This is in contrast to copper or streptomycin products. However, there is no resistance to hydrogen dioxide. So, if one were to apply hydrogen oxide to a greenhouse full of tomato transplants, it might be possible to lower the populations of strains of the bacterial spot pathogen regardless of copper resistance. Due to the lack of residual of hydrogen dioxide products, if the decision to use this product is made, do not leave out a streptomycin or copper application. Rather, use a separate application of hydrogen dioxide. In general, I do not recommend the application of hydrogen dioxide products in the field for control of bacterial spot. (Some compounds of copper, streptomycin and hydrogen dioxide may be organically approved, that is, approved by the Organic Materials Review Institute-OMRI).
Another product that has been used for management of bacterial spot of tomato is acibenzolar-S-methyl (trade name Actigard®). Acibenzolar (ASM) does not have any activity against bacteria or fungi. ASM is known as a systemic acquired resistance product. That is, it ‘tells’ the plant to turn on biochemical pathways that defend the plant from infection. ASM has been used with copper products to lessen the severity of bacterial spot of tomato. However, ASM can cause yield loss if used on tomatoes that are stressed due to drought or other environmental factors. The effectiveness of ASM will not be influenced by whether the bacterial strains are resistant to copper or not.
Serenade Max® has shown activity against bacterial spot of tomato. The action of Serenade Max is reported to be due to a protein component of the bacterial ingredient and to a systemic acquired resistance activity similar to that described for ASM. As with ASM, the activity of Serenade Max® is not related to whether the bacterial spot strains are copper resistant or not. (Serenade Max® is OMRI approved).
The fungicide Tanos® (common name of active ingredients, famoxadone plus cymoxanil) has been trialed for activity against bacterial spot of tomato. While the results have not always been positive, it might make sense to use Tanos® when one is trying to manage one of the fungal diseases on the Tanos label (for example anthracnose, early blight, late blight, Septoria leaf blight) and hope for some activity against bacterial spot as well. The effectiveness of Tanos®against bacterial spot will have nothing to do with possible copper resistance of the pathogen strains.
None of these management techniques, whether cultural or chemical, will be effective as a stand-alone technique for bacterial spot of tomato. Each tomato grower will have to work the management tools mentioned into an overall program. Such a program of bacterial spot management of tomato will include many of the cultural techniques mentioned. Most programs will use copper, Actigard® and work in some of the newer products such as Serenade Max® or Tanos®. The use of some type of streptomycin product as well as copper in the transplant greenhouse is a good idea.
Plant pathologists in industry, academia and government are working on alternative products that may be released someday. It is likely, however, that even these newer tools will only represent a portion of the battle that growers will continue to fight against bacterial spot of tomato.
E.C. Large. 2003. The Advance of the Fungi. The American Phytopathology Society. An account of Professor Millardet and the discovery of copper as a fungicide is just one of the great stories in this book.
Boyd, V. 2014. Rethinking Copper. Citrus and Vegetable Magazine, pages 10-11. This is an account of some of Professor Gary Vallad’s excellent work at the University of Florida about copper making bacterial spot more severe.
Stall, R.E., D.C. Loschke and J.B. Jones. 1986. Linkage of Copper Resistance and Avirulent Loci on a Self-Transmissible Plasmid in Xanthomonas campestris pv. vesciatoria. Phytopathology 76:240-243. This manuscript demonstrates that the trait that controls whether a bacterium is resistant to copper may be located on an easily transmissible piece of DNA.
Marco, G.M., R.E. Stall. 1983. Control of Bacterial Spot of Pepper Initiated by Strains of Xanthomonas campestris pv. vesciatoria That Differ in Sensitivity to Copper. Plant Disease 67:779-781. One of the finding of this manuscript is that the use of the fungicide mancozeb increases the amount of copper available.
The title of this article is pretty scary. But it isn’t entirely accurate. Pumpkins won’t really rot from the inside out. In this article, I will describe one way in which pumpkins can seem to rot from the inside out.
Recently, I was asked to visit a field of pumpkins where the pumpkins were soft and rotting. Some of the pumpkins had already burst. Some were soft and when prodded, the insides flowed out. I set out to try to understand how this could happen.
Although it seemed that the rotted pumpkins were healthy on the outside, upon closer examination, I found lesions of bacterial spot on the outside of affected pumpkins. More information about bacterial spot can be found here.
Most lesions of bacterial spot on pumpkin are scab-like on the surface of the pumpkin. Occasionally, however, such lesions will become infected with secondary organisms such as fungi. When this happens, the lesions may enlarge and cause a rot from the surface of the pumpkin into the interior of the fruit (see photo below).
When I examined the pumpkins that appeared to be rotting from the inside out, all of them had holes that started out as bacterial spot lesions. Frequent rains then allowed water to accumulate in the pumpkins. A variety of microorganisms then caused the pumpkin to rot, seemingly from the inside out. To make matters worse, bacterial spot lesions typically occur on the surface of the fruit where rain may easily enter the enlarged lesions.
There may, of course, be other reasons why pumpkins appear to rot from the inside out. However, look carefully at such pumpkins to see if small lesions caused by bacterial spot have become large enough to allow water and microorganisms inside the pumpkin, causing a rot. While such pumpkins are awful to behold, they are a little less scary if one understands the cause.
This pumpkin has the scab-like lesions of bacterial spot on the surface of the fruit, plus one enlarged lesion that has been secondarily infected. This latter lesion allows access to the inside of the pumpkin fruit where water may accumulate, allowing the fruit to (seemingly) rot from the inside out. (Click to see larger image.)
Please e-mail firstname.lastname@example.org with any questions or comments.
It is perhaps unfortunate that two such very different diseases were given such similar names.
- The fungus that causes powdery mildew forms resistant structures that overwinter in the Midwest. The fungus that causes downy mildew doesn't overwinter in Indiana and must 'blow' in each season.
- Downy mildew has pathotypes of the causal fungus that affects cucurbit species differently. For example, cucumbers and cantaloupe are affected by all five pathotypes. Pumpkins are affected only by pathotype five. Powdery mildew seems to affect most cucurbits without regard to race (watermelon don't seem to be susceptible in Indiana).
- Powdery mildew is caused by a fungus that is related to many other plant pathogenic fungi. Downy mildew is caused by a fungus-like organism that is more closely related to a brown alga. This is why the products that are most effective against downy mildew are often not effective on other cucurbit diseases.
Fortunately, the diseases symptoms are fairly different. At least to a plant pathologist. Below see four photos of downy mildew on pumpkin that I think will help one to tell the difference in these diseases. For more information about downy mildew of cucurbits, see this youtube video
Downy mildew on the upper side of a pumpkin leaf. The lesions are not a
bright yellow, but almost a mustard yellow. The lesions tend to be angular
and the margins of the lesions are definite. (Click to see larger image.)
The underside of a pumpkin leaf with downy mildew. Note that the lesions
appear 'dirty'. This appearance is from the spores of downy mildew that
grow on the leaf bottom under moist conditions. A 10X hand lens will help
to see this growth. (Click to see larger image.)
The most recognizable symptom of powdery mildew is perhaps the white
growth of the fungus (see photo below). Under some ciircumstances,
powdery mildew causes a chlorosis (yellowing). Note that the chlorosis on
this pumpkin leaf caused by powdery mildew is not angular and the margins
are not definite (compare with above photo of downy mildew). The silvering
that can be observed above at some places where the veins meet is not
caused by a disease. Healthy pumpkin leaves often have this feature.
(Click to see larger image.)
In this photo, I have flipped the leaf in the above photo upside down. The
chlorotic (yellow) areas in the above photo correspond to the growth of the
powdery mildew fungus seen here. The powdery mildew fungus can
sporulate on either side of a leaf. (Click to see larger image.)
Downy mildew of cucurbits has been confirmed
in LaPorte and Knox County Indiana on multiple cucurbit hosts. Details of this disease can be found in Issue
588 of the Vegetable Crops Hotline
and in Purdue University Extension Bulletin BP-140W. All cucurbit growers in Indiana area with
developing fruit to protect should consider applying fungicides effective for
downy mildew, some of which are listed below.
However, before spending a lot of money for a fungicide application,
growers should consider that downy mildew does not affect fruit directly, only
the leaves. Those growers with fruit
that will be harvested soon may not notice much if any loss (it might make them
easier to pick). However, if fruit are
developing that have a good chance of maturing, then the cost of a fungicide
application can be weighed against the probable returns of the developing
fruit. Fungicides that are listed in the
Midwest Vegetable Production Guide for Commercial Growers 2014 (ID-56) as effective include: Curzate, Gavel, Presidio, Previcur Flex,
Ranman, and Tanos. Not all of cucurbit
crops are listed on all the listed products.
Read the label for details.
Late blight of tomato has been confirmed in LaGrange County Indiana. In the last issue of the Vegetable Crops Hotline, it was reported that late blight of potato was also reported in LaGrange County. Therefore, tomato and potato growers in the LaGrange County area should carefully scout thier fields for late blight. For more details, see the Vegetable Crops Hotline issue listed above. Also, see the photos below for help in scouting for late blight of tomato.
Late blight on a tomato leaf.
(Click image for larger view)
Late blight on a tomato.
(Click image for larger view)