So much diversity!

Fall is coming to a close, and with it the end of another gardening season. The storeroom is filled with acorn and butternut squashes, onions are collected and potatoes are all dug up. For those of you lucky enough to have a greenhouse or low tunnels the season is still going, but for the rest of us it's time to clean, compost, prepare and wait til next year's gardening begins.

I'll be spending the winter months talking about a slew of topics from varieties in my test garden to insects. The first topic I want to talk about is diversity, and the amazing variation that is available to gardeners.

Apples (bottom) and grapes (top) from around the world in different size, shapes, colors and textures.

This past September I had the opportunity to drive to the USDA ARS germplasm center in Geneva, New York with my good friend Joseph Tychonievich. For those of you who don't know what a germplasm center is, it is a facility dedicated to the maintenance and preservation of genetically diverse plant material. Before commercialization of fruits and vegetables, people selected plants for flavor, texture, color and winter storage, not for traveling, uniform ripeness and some of these other traits that current varieties of fruits and vegetables have to meet. There were hundreds of varieties of melons, tomatoes, cucumbers, carrots, broccoli, etc. Germplasm centers are facilities that maintain the "heirloom" varieties, but they also have wild and unadapted relatives that might not be tasty, but are good sources to breed in disease resistance and other traits. The United States Department of Ag has a number of germplasm centers across the U.S. The center in New York contains apples, grapes, tart cherries as well as some vegetables like tomatoes. In their apple catalog alone there are 100s of cultivars of apples from all over the world as well as wild species from Kazakhstan.

A young apple forest at the germplasm center. You can see the ground is littered with hundreds of fallen apples. Each tree in this block is from a single seed and is therefore genetically diverse from all the other trees around it.

Apples (Malus spp.) originated in Kazakhstan, and in some parts of that country old apple forests still remain. It is from these forests that apples were brought to the rest of the world and selected until they became the apples varieties that we have now. I had two goals when I visited the germplasm center:  1) Try as many apples as I could and 2) Bring back seeds to plant my own apple forest. 

This apple species had beautiful color, the flesh was an orangish red once you cut it open. The taste was a little too bitter for my taste, but it was one lovely apple!

These are all wild species apples from Kazakhstan. The one on the far left is about the size of a Golden Delicious. This was just a sampling from a couple of the trees that were there.

Even the flesh color of these apples had diversity!

Some of the varieties we tasted were amazingly delicious! Others... let's just say they were reminiscent of old gym sock. We tried fruit from related species, that was so bitter and astringent it made your tongue numb. The textures of the fruit were incredible too, with some being mushy and grainy while others were crisp and light. The diversity in apples is amazing. I had no idea that so many varieties existed. There was even one apple variety that tasted almost "meaty" to me.

While I did bring back a lot of seeds, none of these will be the exact same as the apples from which I took them. Some plants are self-pollinated and their seeds will be just like the parent plants, others are only cross-pollinated. Apples are cross-pollinated and each seed is distinct and unique having some combination of genes from its parents. All apple varieties are maintained through cuttings called scions that are grafted onto rootstocks.

Breeding a new delicious apple variety takes a bit of luck and a lot of patience. While none of the apple seeds I planted might end up being the next Honeycrisp, they are a treasure trove of diversity and I'm excited to see what will grow.

This apple seedling came from a Gala I bought at the grocery store. You don't have to go to New York to find apple diversity, it is right in your local supermarket. Remember, apples are cross-pollinated so each seed is unique.

Cucurbit downy mildew

Believe it or not another mildew exists that attacks your cucumbers, squashes and melons. It’s called cucurbit downy mildew. Powdery mildew and downy mildew are two different diseases caused by two very different pathogens. Whereas powdery mildew is caused by a fungus, downy mildew is caused by an oomycete, Pseudoperonospora cubensis. For some of the differences between oomycetes and fungi see the post on late blight.

Cucumber plants with downy mildew in a garden plot. You can see the yellowing foliage.

Similar to powdery mildew, downy mildew is caused by an obligate pathogen that requires a living host. It only affects the leaves of the plant; not the roots, stems or fruits. Don’t be fooled into thinking this won’t do much damage however. It turns out that downy mildew is a huge problem, especially on cucumbers. Commercial growers spend hundreds of dollars each year trying to manage this disease, and it can still cause crop losses. This pathogen is dispersed by the wind and can be blown up from the south or down from greenhouse grown plants in the north. The pathogen infects the leaves, begins to take nutrients away from it. Leaves will die, any fruit that are produced will typically be deformed, and the plant will eventually die.

Close up of a single leaf. The yellow patches on the leaf are downy mildew and will eventually turn necrotic as the disease progresses.

Symptoms

The symptoms differ depending on what type of cucurbit it’s on. On cucumbers you typically see very angular lesions, and they will be restricted by the veins. During early stages of infection the leaves show watersoaking which progresses to yellow angular patches. As the infection progresses those angular lesions become necrotic (dead) and if you look on the underside of the leaf will be hundreds of tiny grayish sporangia. They are too small to see well with the naked eye, but it will look like grayish dirt on the underside of the leaf. On melons, the lesions are more rounded, have a cholorotic (yellow) halo and can easily be confused with other leaf diseases (like Alternaria).

This is the underside of that same leaf. You can see the lesions being restricted by the small veins, and if you look closely you can see the grainy appearance of those sporangia inside the lesions.

Management

Unfortunately there aren’t a lot of cultural (or non chemical) controls for downy mildew. There are no varieties currently available with complete resistance to the pathogen, but some varieties are more susceptible than others. (Ask your local extension agent for varieties that are least susceptible in your area). Start cucumbers indoors to maximize their production time. You can slowdown the effect of the disease by scouting for symptoms. Often downy doesn’t show up until the end of July/early August when the weather is warm (not hot), and humid. Some years, if conditions are good, the disease has been seen in Michigan as soon as the first week of July. The Michigan State University Extension service puts out a notice as soon as downy mildew is detected in the surrounding states. They also have some information for home gardeners, I really like the pictures of the sporangia. The pathogen infects and spreads best in warm humid conditions. So, having good plant spacing and limiting your overhead watering can make the microclimate in your canopy less favorable for disease. If you catch the disease early enough you can remove infected tissue by double bagging it, and putting it in the city trash.

Downy mildew on a cantaloupe. The lesions aren't quite as angular and they have a yellow halo around each one.

So does this mean you should give up on cucumbers? Absolutely not! Start those seeds indoors, space your transplants adequately, scout often, and you'll still have a perfectly decent cucumber harvest.

Source contributing information:

http://ohioline.osu.edu/hyg-fact/3000/pdf/3127.pdf

http://msue.anr.msu.edu/news/downy_mildew_spores_detected_in_michigan

http://msue.anr.msu.edu/news/managing_cucurbit_downy_mildew_in_your_garden

Late blight

When people hear the term late blight, panic often ensues. Late blight brings to mind the Irish potato famine and rotting potatoes. For others, late bight is the kiss of death for their tomatoes. In the U.S. alone, dozens of scientists study late blight and how to manage it. But what is it? And why is it so terrible? Unfortunately, or fortunately, I have never had late blight in my own tomatoes so the only picture I have available is a fruit I found in a research plot.

Late blight is caused by the oomycete pathogen Phytophthora infestans. For those of you who haven't heard that term before oomycetes are almost something between a fungus and an alga. They are more closely related to algae, and are frequently called watermolds, but they have hyphae (those thread-like strands fungi produce) and they also produce tiny spores called sporangia. P. infestans is spread through the air. Its tiny spores are easily caught by wind currents and carried from field to field.

Symptoms

Late blight starts as small watersoaked lesions that rapidly grow,  and turn brown as the tissues die. When conditions are suitable, fluffy white growth can be seen on the dead tissue. Lesions spread, causing leaves and stems to shrivel and die. Lesions occur most frequently on the stems and leaves, but infections can also be seen on the fruit. On green fruit the lesions have a greasy look and are brown. Lesions on the mature fruit can sometimes have a ringed appearance.

A mature late blight lesion on a ripe tomato fruit. You can even see the growth rings.

Disease cycle

Early during the growing season the pathogen infects a susceptible host.  The predominant susceptible hosts for late blight are going to be potato and tomato. The disease cycle begins when a spore lands on a susceptible host and infects the tissues. At first, the pathogen doesn't seem to be dangerous. It steals some nutrients, but nothing much, and just as your plant is realizing that something isn't quite right..... the pathogen takes off. It starts destroying cells, dissolving cell walls, breaking down sugars, and eating the juices inside. Even susceptible plants try to fight back, but the pathogen has too large of a head start. As the disease progresses, large dark necrotic (dead) spots appear on the leaves and stems of the plant, these eventually have white fluffy growth on them. If you could look very closely (even a hand lens isn't enough) you could see little tiny white sporangia. These are the spores that will fly off and infect new plants. The pathogen can overwinter in crop debris, tubers or fruit.

You might be asking yourself, what chance do you have against something like this?

This is where research comes in. Dozens of researchers around the world study P. infestans. How it infects, how plant can resist it, what management practice work, which ones don't work; they even have tracking systems so you can see how late blight moves through an area.  Plant breeders are constantly working to develop varieties (tomato and potato) that are resistant to late blight. Unfortunately, the pathogen is always adapting to infect the plant.

Management

P. infestans can overwinter in the crop debris (potato tubers, tomato stems and fruits) so cleaning your plots is imperative for disease prevention the following year. The only management that traditional gardeners have that organic growers do not is chemical controls. For home gardeners, products like Ortho multi purpose fungicide, Acme Tomato and Green Light Maneb Plus contain effective active ingredients and are labeled for home grower use. If choosing to use a chemical control be sure to read the label and follow all safety rules and regulations for use. All other strategies are in the organic management section.

Organic management

Organic growers have few options for managing the disease once they have it. No organic chemistries or plant extracts have been shown to be consistently effective in controlling late blight. Copper, a staple for most organic management programs provides minimal control. Prevention is really the best way to manage the disease.

Starting with clean materials (tubers or transplants) is the first step. During planting, keeping plants properly spaced will help improve airflow and keep leaves dry. Watering plants from the base vs. overhead watering can reduce moisture on the leaves (Hartill et al. 1990). Similar to many pathogens, P. infestans requires a few hours of leaf wetness before it can germinate and infect the host. If you see the disease while scouting (cite scouting page), removing the infected plants  (double bag and put into the trash NOT the compost), will help reduce the levels of spores. At the end of the growing season be sure to remove all susceptible plant material, regardless of whether it had visible symptoms or not.

Crop rotation is effective for managing late blight, but you will need to rotate with a nonhost for at least 3 years. Late blight resistant varieties of tomatoes exist, and while they are not ideal for commercial gardens, they happen to be perfectly tasty for home gardeners. These include varieties like Mountain Magic, developed by an NC State tomato breeder, and Plum Regal.

Late blight can be difficult to manage, and is likely a disease you hope you never get. With proper scouting, culling and sanitation the losses of this disease can often be minimized.  If you do have late blight, contact your local extension agent immediately for the most up to date (and effective) control strategies for your area. If you don't have an extension agent click here for USAblight, a national site for late blight information.

For Fungicide Options: http://articles.mcall.com/2012-06-29/entertainment/mc-tomato-late-blight-treatment-prevent-20120629_1_late-blight-irish-potato-famines-spore-spread

Rubin E, Baider A., Cohen Y. 2001. Phytophthora infestans producs oospores in fruits and seeds of tomato. Phytopathology 91:1074-1080

Hartill WFT, Young K, Allan DJ, Henshall WR. 1990. Effects of temperature and leaf wetness on the potato late blight. New Zealand Journal of Crop and Horticultural Science 18:181-184

Dorn B, Musa T, Krebs H, Fried PM, Forrer HR. 2007. Control of late blight in organic potato production: evaluation of copper-free preparations under field, growth chamber and laboratory conditions. Eurpean Journal of Plant Pathology 119:217-240

NC STATE Extension bulletin on disease managemenet, chemistries and resistant tomato lines: http://www.cals.ncsu.edu/plantpath/extension/fact_sheets/Tomato_-_Field_Staked_-_Late_Blight.htm

Cornell University Extension bulletin on management and resistant varieties: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Tom_LB_OrganicMgt10.html - Plum Regal

Corn smut anyone?

When your sweet corn has tasseled and those ears are starting to swell and grow, it's a constant battle to keep it free from bugs and animals. Raccoons and squirrels can decimate those ears just before you get to them, and somehow they manage to make it look like the corn is still there. If you manage to avoid them, then there is always the chance that under those husks lurks a corn earworm. However, there is another insidious organism waiting to hijack your corn. It's called corn smut, and it's been written about by scientists for over 100 years. This particular disease looks like some sort of alien growth bursting out of your corn (either the ears or the tassels).

Silvery gray galls of the corn smut on an ear of corn

What causes it?

It's caused by the fungal pathogen Ustilago maydis. This pathogen can infect the ears, tassels, stalks and the leaves of corn. They infect the host (in this case corn) tissue and they cause tumors to start to form. These are light gray/silver to a medium grayish-purple depending on the age of the tumor. The deeper color is the spores that are getting ready to be released.  

Hot dry weather followed by a nice wet period seems to be great for disease formation. However, some sources say that cool conditions and dry weather followed by a rain event can promote disease. Regardless, last year we saw a lot of smut, and, after my last garden harvest, this year seems to be another great year for the disease.

Close up of a smut gall, you can see the dark spores just under the silvery surface

How to manage it

Proper fertilization is important for minimizing disease severity, with higher amounts of disease being seen in over or under-fertilized corn (Aydogdu 2011). If you do get the disease the bet thing you can do is remove the infected tissues BEFORE the dark spores are being released. If the ears are infected you can eat them (see the next paragraph) or you can bag them up and throw them in the trash (not your compost). If some of the galls have black spores being released, go ahead and bag those up and throw them away. You'll need to rotate with a different crop for at least 2 years to manage the disease properly. The Virginia Tech Cooperative Extension office has a good list of sweet corn varieties with some level of resistance/tolerance in one of their bulletins as does an Oregon State University Extension bulletin:

Argent

Brilliance

Fantasia

Pristine

Seneca (Sensation, Snow Prince, Sugar Prince)

Silver (King, Prince)

Summer Flavor 72W

I have never grown any of these varieties, but what I can say is that the variety (I'll let you know what it is as soon as I find the package) I planted this year was quite susceptible. If you want to produce smut however, this might be a great variety to grow.

Fun tidbits

Believe it or not the disease (if you catch it early) is considered a delicacy and is served in many restaurants under the name huitlacoche or cuitlacoche. For those of you wanting to try it, you'll need to harvest it when the galls are still a silvery gray color. Huitlacoche can be used in soups, appetizers, tacos or served plain - here are some recommendations from the University of Illinois. Now before you go grimacing at the thought of eating it, think about it. This is a fungus, mushrooms are fungi. If you'd eat a mushroom, you should be willing to try huitlacoche.

 So if you can avoid getting this disease, that's great! But if you can't, go ahead and be adventurous and make the best of it.

Aydogdu M. and Boyraz N. 2011. Effects of nitrogen and organic fertilization on corn smut (Ustilago maydis DC Corda.). African Journal of Agricultural Research 6: 4539-4543

Skibbe DS., Doehlemann G., Fernandes J., and Walbot V. 2010. Maize tumors caused by Ustilago maydis require organ-specific genes in host and pathogen. Science. 328:89-92

Powdery mildew

Powdery mildew is probably the most common vegetable disease urban gardeners encounter. Those powdery little spots appear mid summer, and by the end of the summer they’ve covered entire leaves with their fuzzy white powder. While you might not be so sad to see those overly prolific zucchini decimated, the cucumbers and pumpkins won’t be far behind. So what causes this disease? Why do we get it every single year? And what can we do about it?

What causes it?

Powdery mildew is caused by a fungal, “obligate” pathogen. So what does that actually mean? An obligate pathogen means that it requires a living host, in this case a plant, to reproduce and survive.  Think of it like this: the pathogen wants to hijack the plant's resources for its own benefit, instead of producing those delicious vegetables. This lifestyle strategy is going to affect how we manage the disease and how it infects your plants. While powdery mildew is most common on cucurbits like cucumbers, squashes and melons other plants such as tomatoes, onions, peppers, chards and even sugar peas. However, many of these powdery mildew diseases are caused by different fungal species. Because powdery mildews are obligate pathogens this often means they have a very specialized relationship with their host plant. The powdery mildew on your squash is going to be different than the powdery mildew on your tomatoes, and it can NOT go from one to the other. I'll be providing information about powdery mildew management in general, but some specifics might not be appropriate for all species.

Where does it come from?

So I’ve just told you that powdery mildew is an obligate pathogen and it requires a living host. You’re first thought is probably, “If it requires living plant tissue, and isn’t living in the soil over the winter, how does it show up every year?”  Powdery mildew is an airborne pathogen. It’s spread as air currents flow across the U.S. Just because places like Michigan have winter, not all places do. Powdery mildew can survive in those places, and then as the weather warms it can slowly creep its way farther north. Even if we could somehow prevent it from moving up from the south, greenhouse grown plants could be another source of the pathogen. 

Early symptoms of powdery mildew. The small white spots can be rubbed off (but don't rub them otherwise you can spread the pathogen).

What does it look like?

The symptoms of powdery mildew are pretty much what you would expect. They start as small white powdery spots, sometimes they’re also a bit chlorotic (yellow), on the upper sides of the leaf and as time progresses those spots will expand and spread. Often lower leaves show symptoms first.

Conditions that favor the disease

Powdery mildews like high relative humidity, without a lot of a leaf wetness (Granke et al 2012). This is one of the reasons the first leaves to show symptoms are generally the lower leaves. The lower leaves are more protected from rainfall and are buffered by the surrounding leaves so they often have a higher relative humidity than upper leaves in the canopy. This is one reason why adequate plant spacing is so important for minimizing diseases. Low humidity promotes spore dispersal (like tiny airborne seeds), while the high humidity promotes infection (Clemson). Temperature can also be a factor - powdery mildew does not like it too hot, so those hot hot days are not going to be good for the pathogen.

Later symptoms of powdery mildew. This shot is of a leaf that was lower in the leaf canopy than the top picture. I moved the top leaves aside and hiding underneath was this plethora of powdery mildew spores.

Management

While prevention is the best way to manage a disease, at this point in the growing season minimizing the effect of disease is the next best thing. Whether you want to use chemical sprays or organic methods, early detection of the disease (through scouting) is critical for successful management. Studies have shown that sodium bicarbonate (baking soda) and whole milk can help minimize the growth and spread of powdery mildew. Baking soda, when used at a rate of 0.5% (weight per volume) combined with an oil, like SunSpray Ultra-Fine Spray Oil (also at a rate of 0.5%), reduced disease severity when sprayed on the plant's leaves (Ziv and Zitter 1992). Commercially available products like Kaligreen (82% potassium bicarbonate) can also help suppress the disease (Mcgrath and Shishkoff 1999). Whole milk, not skim, reduced disease severity at a higher rate than just baking soda in a more recent study by Ferrandino and Smith (2007).  The authors used a 50% milk to water spray, and did state that good coverage was needed, however that the cost may be prohibitive for large areas.

While these are options that should be readily available in most households, these are not as effective as chemical fungicides when the disease severity is high. So if you plan to use these products then you should definitely be careful to scout frequently and do your best to catch it early. If the infected leaves are few and far between you could easily remove a few leaves here and there. But likely you will have latent infections (when the plant is infected but you can’t see the disease symptoms), and you’ll have more diseased leaves soon.

Don't confuse variegation with powdery mildew. This squash has variegated (patterns of light and dark) leaves, but no powdery mildew.

Remember, powdery mildews need a living host. This means that the pathogen’s goal is to make your plants make food for them instead of you. Your goal is to keep ahead of the pathogen so that your plants can spend all their photosynthetic energy on making those delicious veggies you want to eat.

References contributing information in this post:

Clemson University Cooperative Extension bulletin

Ferrandino F.J., and Smith V.L. 2007. The effect of milk-based foliar sprays on yield components of field pumpkins with powdery mildew. Crop Protection 26:657 -663.

Granke L.L., Crawford L.E., and Hausbeck M.K. 2012. Factors affecting airborne concentrations of Podosphaera xanthii conidia and severity of gerbera powdery mildew. Hortscience 47:1068-1072

Mcgrath M.T, and Shishkoff N. 1999. Evaluation of biocompatible products for managing cucurbit powdery mildew. Crop Protection 18:471-478.

Ziv O., and Zitter T.A. 1992. Effects of bicarbonates and film-forming polymers on cucurbit foliar diseases. Plant Disease 76:513-517