Some like it hot(ter)

Item: Sacramento's tomato season shaping up as short, sweet
By Debbie Arrington, Sacramento Bee, Aug. 11, 2010
From home gardener to commercial growers, tomato season has been especially challenging this year. "It's August but feels like May," said Suzanne Peabody Ashworth, who grows more than 60 varieties at her Del Rio Botanical farm in West Sacramento.
"This has got to be the latest tomato season I've ever seen. I can't remember a summer quite so cool."
Item: Temperatures continue well below average in Southern California
By Hector Gonzalez, Staff Writer, Whittier Daily News, 08/09/2010
It hasn't been the coolest summer on record, but it's been close, forecasters say. The average temperature in July was 79 degrees, five degrees below normal, and the first eight days of this month also have been five to six degrees below normal, weather experts said.
"The fruits and vegetables, the tomatoes and a lot of the citrus and things like raspberries are not ripening up because it's not getting hot long enough," [said an expert from the LA county arboretum], adding that some fruit could taste less sweet because "less sun means less sugar content."
Why aren't my vegetables producing?!
The most common question we're getting these days has to do with poor fruit production on beans, melons, squash, and tomatoes, among others.

Tomato inflorescence
What's going on? This bloom cluster on a Roma tomato illustrates the problem. Tomato flowers open gradually along the cluster toward the end. The first, oldest flower set fruit. Temperatures when the next two flowers opened were very cool at night, with one night dropping to 48 degrees! Those blossoms have not set, and will fall off. The bloom currently open will probably set. So this cluster is setting fruit on about 50% of the flowers.
This may not be the coolest year on record here, but every month of 2010 has been substantially below average. The Delta breeze has been the dominant regional climate effect. We've been having Bay Area weather: as I drive in to Davis from Dixon each morning, I can see the layer of fog sitting nearly on Fairfield to the west every morning. When was the last north wind?!?
After a very late start getting vegetables planted due to persistent rain and chilly weather, growth and fruit set has been very slow. Most tomato plants are setting pretty well, but fruit has been ripening 4 to 6 weeks behind schedule. While we still have plenty of time for fruit to ripen, but yields will be substantially lower than usual. Get out grandma's old green tomato recipes!
In an average summer, gardeners are concerned about the effect high temperatures have on fruit set of vegetables. Anytime the daytime high is above 85 - 90 degrees, flowers fail to pollinate and drop off. Ordinarily enough of our days top out below 90 that we get adequate fruit set, and the warm days encourage fruit development and ripening. Usually harvest begins in July and continues through October.
This year? In the first 75 days since June 1, 60 had below average high temperatures, by an average of more than 7 degrees. Only 10 days have been above average, and we've only had 3 days at or above 100 degrees this summer! High temperatures haven't been the problem this summer.
A less common problem for us, usually, is the effect of cool nights. Temperatures below the mid-50's reduce fruit set, but that is usually rare in the Sacramento Valley. Not this year! In those 75 days, 58 nights were below average. 30 have been 52 degrees or colder. More than half our nights have been unsuitably cold for fruit to set. Add poor pollination to the mix, and you have a recipe for poor yields in Davis gardens this year.
If my tomato plants are any indication, there will be a reasonable crop in September and October. But yields appear to be 20 - 50% lower than normal, varying by variety. Fruit tree varieties are ripening behind the ripening charts by as much as 3 to 4 weeks. I am getting some comments that fruit "isn't as sweet as usual."While that would make sense due to less heat input, it is also very subjective and not provable one way or another.
Some other problems from our odd summer weather.
While gardeners have been enjoying the cooler gardening conditions, it has increased other problems.
Fireblight! This bacterial disease attacks and kills the blossoms and tender growth of certain members of the apple branch of the rose family, and it's been especially bad this year.
Specific host plants include apples and pears, including ornamental pears; hawthorn (the tree Crataegus, not the India hawthornRaphiolepis), and pyracantha. Contrary to what you may read on internet sources, fireblight does not attack rose bushes.
The most common symptoms are blighting and death of blossoms of fruiting apples and pears, and rapid dieback of young shoots. Fireblight has a narrow temperature range of about 55 to 80 degrees, which coincides with the bloom period of those fruit trees (March to April). Bees with dirty feet spread the bacteria from bloom to bloom. Once we get above 80 degrees consistently, the bacteria stops spreading.

Fireblight on an apple tree
Fireblight is caused by a bacterium called Erwinia amylovora. It attacks specific members of the pome group of the rose family, particularly apples and pears, infecting through the blossoms or the young tender growth. The disease kills the flowering wood and spreads rapidly down the stem. The common name refers to the fact that it blackens the wood as it kills the shoots and branches, so the plant looks as though it has been burned. Prune it out!
Temperatures during bloom were actually lower than ideal for fireblight. But fireblight can also attack soft tender growth and cause rapid dieback. Cool conditions in the suitable infection range continued here through April and May. While our usual infection period is just a couple of weeks, we had continued infection this year for nearly two months. Resistant varieties that get infected will show some flagging (dieback of short shoots) before the progress of the disease stops. Susceptible varieties may lose entire branches or even be killed. Prune out fireblight as soon as you see it!
When you choose a variety of pear, it is worth considering fireblight resistance. Bartlett, the most popular commercial pear, is very susceptible; Moonglow is resistant. We get fireblight every year, and it is hard to prevent. Copper sprays during the bloom period may be helpful. It is important to prune out dead infected portions now, as those are sources of reinfection next season. They're easy to spot. Just prune down below the blackened portion into healthy wood. Dispose of the prunings in the trash rather than the compost pile. Contrary to popular literature, it is not necessary to clean your pruners with bleach as you cut.
Powdery mildew, particularly on sycamores and plane trees.
Sycamore and plane trees (Platanus species and hybrids) are among the most common species in downtown Davis and other neighborhoods, and for good reason: they grow fast and provide quick shade, with very few pest problems. But anthracnose blight attacks the leaves in spring, causing them to drop, and then powdery mildew causes leaves in early summer to curl. The leaves are disfigured and covered with white powdery strands and spores (hence the name).

Powdery mildew on a plane tree
We always get some mildew in early summer, but periods of hot weather usually stop the continued spread. Not this year! Mildew doesn't really harm a healthy, vigorous sycamore tree, and there is no practical treatment for a large plant. Powdery mildew is host-specific: the type you get on your sycamore tree is different from the type you get on your rose bushes. So you needn't worry about cross-infection in your garden. If you are selecting a Platanus, look for hybrids that are resistant. Bloodgood is resistant to anthracnose, but not to mildew. Columbia variety, becoming more common in the trade, is resistant to both anthracnose and mildew.

Disease resistant Platanusvarieties

The best way to deal with disease problems is to prevent them in the first place. Sycamore leaves attacked by powdery mildew (photo, left) are curled and distorted and covered with white powdery material; leaves attacked by anthracnose blight fall off after spring rains. Two varieties ofPlatanus (above) have good disease resistance. The Roberts variety of California sycamore (P. racemosa, above left) resists anthracnose. Columbia variety of plane tree (P. x acerifolia, above right) resists both anthracnose and powdery mildew.
On the plus side, we're seeing fewer problems with crown rot on woody plants. Caused by Phytophthora, this disease attacks shrubs and trees when they are over-watered during very hot weather. Native plants and other species from regions with Mediterranean rainfall patterns are especially prone to it. Phytophthora requires very high moisture and high soil temperatures and is especially encouraged by daily watering at or near the crown of the plant. This year's cooler soil temperatures are probably reducing the problem.
The flip side of that is that we are seeing a lot of nutritional deficiencies in plants indirectly caused by watering too often. Water use by plants is lower than usual this season. When you damage the fine root hairs with constant moisture, their ability to take up trace elements such as iron, magnesium, and zinc is reduced. Symptoms of these deficiencies include yellowing of the leaves while the veins remain green (iron), blotchy yellowing (magnesium), and leaves that are simply smaller than usual (zinc). We're getting a lot of calls about citrus trees with these deficiencies, which are pretty easy to correct by applying specialty fertilizers. But if you are watering daily, that is part of the problem. Watering slowly, deeply, and infrequently will reduce damage to the roots.
As they say, there is no such thing as a normal year; just average (or not). Below average temperatures = below average yields, plus a few other problems. But the gardeners themselves seem to prefer these milder conditions.

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Written for the Davis Enterprise, August 26, 2010