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