I recognize the picture.

However last year I didn't know to ask if the white shit on the zuke leaves is the "mildew" in question. I just thought (never having grown the stuff myself) that maybe zucchini leaves were green-and-white in normal structure. Shows what I know.

(Of course I can't even figure out why what looks like either a watermelon or a hand grenade is growing on my cucumber vines. I suspect either alien invasion or the NSA. Bastards.)

And you will notice that vastleft has been....vewwy vewwy quiet since I left that comment. Downright missing one might even say. Vasty will no doubt come back with some claim of having been busy with a RL project or some such baldersash. We know better. :)

there are two mildews here to watch out for... powdery mildew (which occurs in dry conditions) and downey mildew which occurs in moist cloudy humid conditions (especially early in the year).

In either case never top water plants especially at night in the case of downy mildew. Powdery is a little easier to control because base watering/irrigation of your plants and mulching can keep mildew away in dry conditions. Downey is more annoying because of the wet and humid moist air, rain, etc... in that case try to give the plants as much air circulation as possible. One solution I've read about, for downey mildew, suggests spreading additional compost and cornmeal on the soil surface under the plants. I've never tried it though... and its almost always powdery mildew that gives me problems later in the summer.

Another home remedy solution to rid powdery mildew - besides milk (which i'd never heard of) - is spraying with baking soda. 1 tblspoon baking soda to 1 gallon water 2-3 drops of liguid detergent and veg oil to make the solution stick... spray on leaves over and under... until mildew stops. Its tedious but it works if the mildew isn't too far advanced. Once you tire of all that (and take my word for it you probably will)

... you can try the commercial products (some are organic). See some organic examples here: here

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Sphaerotheca fuliginea (Schlecht.) Pollacci

Reference:

Powdery Mildew by Allen Buchinski – October, 2003
http://www.mastergardeners.org/publicati...

“Symptoms most often appear in the form of circular white spots; the spots enlarge as the white mycelium (threadlike filaments) grows over plant surfaces and produces spores, giving a powdery appearance that covers the leaves (although one species that affects onions, peppers and tomatoes has a yellow appearance with little powdery growth). The infection forms on either side of the leaves, spreading rapidly over the entire surface with the mycelium sending a root like structure (called haustoria) into the leaves, taking nutrient from its host.”

“Infection is favored by high humidity (50 to 90%) during twilight hours, and temperatures between 60° and 80° F. While there’s some ambiguity in the UC IPM literature, most sources state that not only does PM not require moist conditions to germinate and grow but that the spores can be killed or inhibited by water on the plant. Shady conditions (which are most conducive to retaining humidity) and vigorous plant growth favor disease development while high temperatures (above 90°F) have an inhibitive effect.”

Just to be absolutely certain of what you’re dealing with, pull out the Sherlock Holmes cap and magnifying glass:

“A side note: don’t confuse PM with downey mildew, they’re different diseases, with different symptoms and control mechanisms. While downy mildew can also produce a powdery growth, it grows primarily on the underside of leaves, and when viewed with a hand lens, has a tree like structure instead of the threads of PM. Downey mildew infections are favored by moist humid summers (high humidity with moderate temperatures), making it more of a problem along the coast where it can cause serious problems for plants including roses.”

Results of investigation and weekly progress reports are expected.

[rimshot; laughter]

I think my variety is caused by (1) wetness (last summer we has 30 days of rain in June), (2) planting too close (the lady in the Post Office said the only solution was a fan; she meant to keep them dry, but I think that's a surrogate for planting them farther apart); and (3) as farmer writes, top-watering. I did that last year, when I thought the leaves were wilting because they were too dry! The problem then exploded. So, from the behavior, I think I've got downey, not powdery. Tomatoes are not affected; it's broad leaf (?) plants like the pumpkins and the squash are vulnerable. Also, the underside isn't affected. What happens is that the leaves curl from the edges inward, then turn brown and shrivel. "I never saw a garden die so fast," the lady from the church commented last year.

So, I went out this morning, after the milk-spray, and the leaves aren't any worse off than they were, and may be slightly better (not quite as limp. That may be the earliness of the day, of course). Since this stuff spreads rapidly, that's good news.

I like farmer's cornmeal and compost suggestion. I'm not sure if the theory of the cornmeal is moisture-attracting, or nutritional. I suppose the theory would be that there are three things going on: (1) is the soil, which creates the conditions for the healthy plant, (2) is the adaptability and presence of micro-organisms, and (3) is the "flesh" of the plant, as it were, whose surface we treat if the soil conditions haven't kept the plant strong enough to resist the micro-organisms. I remember reading one theory that the reason milk worked was not microorganisms that ate the mildew, but calcium! So I think what I'm looking for is something in the soil to make the "skin" of the plants more resistant to the downey mildew lodging in the pores... Along with more spraying with milk.

Yeah, the spraying is a pain, but my garden isn't that large. And it's a lot nicer to spray using my bare hands and in my bare feet, as opposed to wearing a moon suit. The warnings on the copper powder were terrifying.

Lot of great metaphors here for the larger politics, conceived as a network of information and power flows where persons are the nodes...

No authoritarians were tortured in the writing of this post.

especially fungal problems, are best dealt with by the victims themselves. Prudence dictates that those not directly affected keep a certain emotional as well as physical distance; literally sticking one’s nose into a fungal infection is always unwise. Others can offer guidance but in the end, you are the one that has to make the hard choices.

I am nevertheless going to get Fristian here and say that for a long list of reasons it sure looks and sounds like POWDERY mildew to me. Before you invest any more time on treatment, do as The Farmer suggested above and sort out definitively which fungus you’re dealing with. Do the diagnosis and please get back to us, now that there’s a throng of concern.

Here’s the differential:

Color: Powdery mildew is initially white, turning grey as it produces spores. Downey mildew can vary but usually is initially yellowish, turning purple or black as it produces spores. If you can’t tell immediately, tear off an infected leaf, wrap in moist paper towel and seal in a ziplock in the fridge overnight. Color should become apparent in at most a day.

Initial site of infection: Powdery mildew dominantly starts on the top surface of established leaves. Downey mildew always starts on the undersurfaces, although you may see signs on the top surface first. Turn over an affected leaf in the early stages. If the most extensive spread is on the top, it’s likely powdery; if on the bottom, it’s likely downey.

Shape: Initial infections with powdery mildew are more or less circular, with spread into varied but generally rounded shapes. Downey mildew infection and spread is limited by the vasculature and the shapes are angular, typically restricted to leaf segments between major veins. See photos below.
Photos and descriptions of cucurbit mildews:

Powdery mildew at http://vegetablemdonline.ppath.cornell.e...

And http://www.ext.vt.edu/pubs/plantdiseasef...

And because SEMs are always cool http://denniskunkel.com/DK/Fungi_and_Sli...

Downey mildew can be seen at http://vegetablemdonline.ppath.cornell.e...

And http://vegetablemdonline.ppath.cornell.e...

With regards to top watering, there are two distinct issues: initiation of infection and spread of infection. Powdery mildew infections are NOT encouraged by top watering; in fact, controlled scientific studies have shown that top watering INHIBITS infection probably by washing off of the spores. Other studies have shown that top watering has NO EFFECT whatsoever on initial infectivity of downey mildew. For both types, on otherwise healthy plants it’s all about humidity, temperature and prevalence of spores. Once an infection is established, top watering can spread the spores but so can wind, people or animals walking through the patch, or the evil gnomes that creep through our gardens at night (shhh, they can hear you talking.)

Species selective infection is typical. As different cucurbit species evolved, the powdery mildew parasite evolved along with them in a species-specific manner. In the US, what we call powdery mildew is almost exclusively caused by Sphaerotheca fuliginea of one or another variety, each limited in its effect to a single host species. The fungus thriving on your zucchini cannot infect your tomatoes.

The role of calcium in mildew susceptibility is like vitamins and minerals in humans. A deficiency can increase the likelihood of disease, but when present in sufficient amounts there is no benefit to further addition – Enough is enough, more than enough is a waste, or toxic.

Lots of people say lots of things. Ask yourself; are postal workers as a rule really your best source of information on any topic beyond postage due? I’m just grateful each time I get through the line and back outside without being subjected to gunfire.

As much as I enjoy the topic, further discussion by me on the general nature and effects of pathogens will be by request only, pretty gruesome and in my personal experience there is no better way to clear out a group at a cocktail party or put a chilly end to an otherwise promising date. For a prescient, hard science view of the future course of human-pathogen interaction read The Coming Plague by Laurie Garrett; fair warning, this excellent, intelligent and accurate book will suck the joy out of even the sunniest day, and is not recommended for bedtime reading or for the persistently depressive. Those with a taste for the macabre will be thrilled.

It’s all in the science, Lambert. Faith-based treatment of fulminate infectious disease seldom ends well.

the problem with top watering is that it can stir up the powdery mildew fungus in the dry soil underneath the plants - what they call the rain splash effect (essentially causing the spores in the dry soil to be splashed up into the air and spread to the plant leaves etc....The water itself isn't the problem in the case of powdery mildew... its the dry soil under the plants (where the fungus lives) combined with warm and or humid air conditions as well. So keep the soil under the plants moist to avoid powdery mildew fungus from growing in dry soil underneath the plant. Mulch under the plants too. That helps keep the soil moist and cool and less appealing to powdery mildew development and the spread of spores into the air and onto the leaves.

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the problem with top watering is that it can stir up the powdery mildew fungus in the dry soil underneath the plants -

or maybe not. i'd always thought PM were a soil born thing that thrived in dry soil and was spread to the leaves where it developed but after reading some more i guess that may not be so much the case. Now i'm confused too, although hot humid dry conditions seem to favor PM.

this from cornell link provided above by bringiton:

Powdery mildew develops quickly under favorable conditions because the length of time between infection and the appearance of symptoms is usually only 3-7 days and a large number of conidia can be produced in a short time. Favorable conditions include dense plant growth and low light intensity. High relative humidity (RH) is favorable for infection and conidial survival, but infection can take place at RH levels as low as 50%. Dry conditions are favorable for colonization, sporulation, and dispersal. Rain and free moisture on the plant surface are unfavorable, however, disease development occurs in both the presence or absence of dew. Infection can occur at 50-90°F; mean temperatures of 68-80°F are favorable. Powdery mildew development is arrested at daytime temperatures of 100°F or higher. Plants in the field are often not affected until after fruit initiation. The leaves are most susceptible 16-23 days after unfolding.
cornell

And this from Colorado State page:

Soil-borne pathogens such as Fusarium (Fusarium root rot and Fusarium wilt of dry bean) and Macrophomina (ashy stem blight of dry bean, charcoal rot of corn) are usually favored by drought conditions that stress plant roots and predispose them to colonization and infection. Foliar pathogens such as powdery mildew (of sugar beet, pumpkin) are also favored by extended periods of low moisture and relative humidity. Drought conditions discourage the survival and activity of other soil-borne pathogens and most foliar-infecting pathogens that require high moisture conditions for survival, infection and disease development. - Colorado State

And on and on it goes.

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that powdery mildew would be pretty well sorted out and clear but you'd be wrong, eh Mr. Farmer? My experience, which of course I know to be more authoritative than that of anyone else, including gardens in Utah, Colorado and a dozen different California microclimates, is that the stuff always takes hold when nights get warm and humid. Could be coincidence, dunno. Dry conditions stress the plant as well as let dust kick up easier, a bad combination for infestations of all kinds.

I used to avoid topwatering too but I don't worry about it anymore - after I noticed that no one goes running out to tarp their veggies when it rains. It is important I think to not top water late in the day, so the foliage has a chance to dry before nightfall. Why? I just do.

Nice having an exchange with you about Lambert's mildew, surely he's hanging on every word. I picture him out there right now on his knees, examining spore shapes and damage patterns, the definitive identification will be posted here any moment.

You Would Think In This Day And Age
that powdery mildew would be pretty well sorted out and clear but you’d be wrong, eh Mr. Farmer?

Yes, exactly my thoughts too after reading around all over the place on this topic.

I always followed the no top water rule (although most of what i read still suggested not to top water especially in hot humid conditions) and i also almost always have trouble with the powdery mildew later in the season when nights are warm and humid (like August) and/or hot/humid and dry. I always figured rainfall during the late summer, when it was hot, humid, and the soil prone to be dry or drying out more easily, was the reason the PM appeared that time of year. But after reading around it seems as though the PM occurs pretty much at any time under almost any kinds of circumstances. I also till in composted leaves sometimes and i'm wondering now if the PM spores overwinter on the leaves? But, still, I dunno. Usually if I get the powdery mildew on zukes it almost always occurs in late summer to early sept. About the time I never want to see another zucchinni as long as i live. I've never had a problem with PM on cukes or on most squash including butternut varieties (perhaps because of disease resistant varieties). Have had a lot of trouble with it recently on all kinds of pumpkins though. Again, usually appears in late summer to early fall. So I dunno. This article (snip below via cornell) was interesting although i have really no specific idea what the hell they are actually talking about -- what with all the pycnidias and conidiophores and immature cleistothecia running around willynilly -- and its specifically referring to PM pathogens on grapes -- but its interesting:
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Life Cycle
The pathogen overwinters or survives intercrop periods as pycnidia. The relative importance of pycnidia within different host structures (i.e., hyphae vs conidiophores vs cleistothecia) as survival structures is unknown. When it rains, conidia are exuded from the ripe pycnidia and are dispersed by rain-splash to mildew colonies. Free water is required for infection, and infection is favored by warm temperatures (20-30° C). Under favorable conditions, infection can occur in less than 24 hours.

The mycoparasite directly penetrates the walls of hyphae, conidiophores, and immature cleistothecia, but may be unable to infect mature cleistothecia. For approximately 7-10 days, the mycoparasite spreads within the hyphae of the mildew colony without killing it. Thereafter, the process of pycnidial formation begins, and is then completed within 2-4 days. Infected cells generally die soon after pycnidial formation begins. Secondary cycles of infection result from conidia released during subsequent rain events.

Several generations may occur during a single growing season.
http://www.nysaes.cornell.edu/ent/biocon...
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It is important I think to not top water late in the day, so the foliage has a chance to dry before nightfall. Why? I just do.

Yeah, I think that too.... and I think that your reason for thinking so is as good as anything I've read. Basically this seems to be another one of those trial and error - what works best for you in your given microclimate location - kind of solutions. I think.

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Doesn't matter where I go or what I do, PM shows up sooner or later on everything susceptible. It is possible that I am the reservoir host. The milk spray helps, doesn't wipe it out but holds it back.

I send infected parts off with the greens recycle, but even moderate infestation would so load the soil it probably doesn't matter. Ritual behavior.

Lovely technical article, fabulous usage of terminology, classic of the genre, thank you. I feel smarter just for skimming through it.

While, ahem, waiting for Lambert to get back with an identification I took a closer look at his photo and the spots do look pretty angular, a pattern more typical for downy. On the other hand, they also look white and he called them white, which would argue for PM. On the other other hand, if it’s downy the underside should look like moldy bread, surely obvious enough to mention.

Perhaps Lambert has a mutant mildew that can be used as a bio-weapon to defeat Xan’s mutant cucumbers should they, in true sci-fi horror genre fashion, turn out to be considerably more dangerous than the benign creatures they now appear to be.