Powdery Mildew

By: Hannah Todd, Home Horticulture Coordinator for UMaine cooperative Extension Piscataquis County. Adapted from an article written by Somerset County Master Gardener Volunteer, David Heroux.

All in all, I consider this to be a perfect summer, especially after the summer of rain in '09 and summer of high heat in 2010. But, yes there is always a "but", as a gardener I know all too well that there is no such thing as a perfect summer (growing season). This season I've had little complaint other than, the hordes of Rose Chafers, Striped Cucumber Beetles, and earlier in the season, the indulgent slugs. So I actually had to ponder what would make for a informative article for the August newsletter, but that's when I remember what I've noticed most this season, which is powdery mildew on cucurbits (squash & cucumbers, etc.).

Signs

This disease starts out as small yellowish/white spots on leaf surfaces. Powdery Mildew can spread to cover the entire plant. Severe infections cause the leaves to turn brown and drop. The fungus, is not typically fatal, but will weaken the plant, resulting in poor yields and diminished growth.

Powdery Mildew on Squash

The Organism

There are several different fungi that cause Powdery Mildew, although the fungi appear similar and have a similar culture they are different species. Different fungi infect different plant species, this helps explain why one plant may be infected in the garden, but not all. For example: Microsphaera alni infects lilac, while cucurbits (cucumbers, squash, etc.) can be infected by either of two fungi, Sphaerotheca fuliginea and Erysiphe cichoracearum. Also it is the Sphaerotheca sp. That will infect Phlox, roses and strawberries.

Powdery Mildews Life Cycle

The disease overwinters as spores and fungus strands on decaying leaf, stem surfaces and in dormant buds of woody plants (Maine is (in most cases) too cold for the fungi to overwinter). In spring spores are dispersed by wind and rain which can carry sexual spores from very far away. In summer the spores infect buds, leaves, and fruit and then mildew appears on the plant. The disease spreads during the season by asexual spores. Late summer into fall most of the fungus species that cause powdery mildew form new resting structures known as cleistothecia.

Treating Powdery Mildew

What's that old saying, "prevention is the best cure", hence, avoid the disease altogether by planting resistant plant varieties and space plants accordingly. Early detection can be the key for reducing powdery mildew damage. Removing and destroying infected areas of the plant as soon as the signs are observed. Furthermore, thorough fall garden clean-up will help reduce powdery mildew for future seasons. Note: infected plant material should not be placed in the compost pile, as home gardener's compost piles don't often reach desired temperatures to kill off these pathogens (instead they can provide a "warm" place for the pathogen to over winter).

Environmental Conditions

Powdery mildew prefers warm, dry weather with relatively high humidity. So follow plant spacing requirements and if dealing with a plant that is known for its susceptible to the disease, such as phlox, plant it in full sun with as much air circulation as possible. An interesting feature of these organisms is, unlike most fungi the fungus species that cause Powdery Mildew do not require wet leaves or soil surfaces for germination.

For more information of powdery mildew on cucurbits please visit:

http://umaine.edu/ipm/ipddl/publications/5085e/ 

Garlic Maturation, Harvest and Curing

an excerpt from the Maine Garlic Project Newsletter by UMaine Extension Garlic Experts Steve Johnson and Dave Fuller

Since picking the scapes from your garlic, the plants have been putting all their energy into bulb production. Bulb formation is triggered by long days and good-sized garlic bulbs are produced by having healthy, large tops without scapes. Garlic bulbs are really only sizing up in about the last seven weeks of the plant life cycle prior to harvest, so you can see the need for weed control all along, but especially in July, when it's easy to get distracted by other crops in the garden.

So when do you harvest garlic? If harvested too soon, the bulbs will not have developed fully and will not store well. When harvested too late, the wrapper on the bulb break down and the bulb opens, exposing the cloves to soil microorganisms, which discolor the cloves, making the garlic unattractive and not suitable for gifts or sale, though they are still fine to eat.

When a freshly harvested garlic plant is deconstructed, you'll see that each leaf corresponds to a wrapper on the bulb. Garlic plants typically have about 7-9 leaves, depending on the variety, with the bottom leaves being the outer wrappers on the bulb.

In determining when to harvest your garlic, count the leaves from the bottom of the plant. The bottom two leaves should be dead and brown (it's easy to overlook the lowermost leaf - very little of it may be remaining) and the next two leaves going up the plant will look brown or very yellow. The uppermost leaves will probably look at least partially green with yellow or brown tips. If you have left a few scapes on your plants, you'll notice that they have curled, uncurled and straightened up to indicate approximate harvest time, as well.

When harvesting garlic, dig underneath the roots to loosen the soil - a spading fork is handy. Brush the loose soil off and don't wash the bulbs. One option is to let the pulled plant dry for up to a day or two in the field. Leaving the bulbs longer than two days in the field may cause sunscald. After field drying, the harvested plants can be transferred to an airy place such a garage to cure. Another option is to directly transfer the harvested plants to the curing area. Curing may take several weeks. At this point, harvested plants can have all but 6 inches of the tops removed or the tops can be left on as options. When curing garlic, make sure not to pile it, as it will mold. Also, it is not necessary or desirable to cure in a very hot space like an attic. After the bulbs are cured (the roots will be very dry), the stalk can be trimmed down to within an inch or so from the top of the clove. My approach is to directly transfer the harvested plants to the curing area, cut off the stalk six inches above the bulb, and immediately cure them. When the bulbs are cured, I trim off the stalks to an inch of the top of the bulb and trim off the roots.