Could a Cherry Tree Tent be the Answer?

 We've posted before about a huge threat to backyard cherry trees in our area: the cherry maggot, also known as the Spotted Wing Drosophila. SWD is responsible for the little worms that backyard gardeners are starting to find in their cherries, and to a lesser extent in raspberries, strawberries and blueberries. An experiment conducted at the Fair Oaks Horticulture Center this past year by Sacramento County Master Gardeners may provide an answer to this growing problem throughout the West.

This was originally published in the Sacramento County Master Gardener Newsletter of Sept. 2012.
 
The problem: Spotted wing drosophila (SWD), is a gnat-sized fruit fly first found in 2008 damaging fruit in many California counties. Unlike the common vinegar fly that attacks rotting and fermenting fruit, the SWD infests ripening, undamaged cherries as well as ripening raspberry, blackberry, blueberry, and strawberry crops – especially in coastal areas.
 
Prior to 2011, no SWD damage was found on the multi-grafted cherry tree at the Fair Oaks Horticulture Center. In 2011, the early varieties escaped damage, but the later varieties were totally infested. In an attempt to prevent fruit loss in 2012, and to demonstrate to homeowners a possible cultural means of preventing infestation, the orchard group decided to tent the tree.
 

The tent: Mary Kay Ryan and Patty Peterson designed a structure to cover the tree while still allowing the harvest of seven varieties of cherries ripening at different times. The tree was pruned to just under 10 feet. Mary Kay and Patty used 1” schedule 40 PVC to create a large box frame anchored to the ground at the corners. The tent itself – 13.6 feet wide, 13.6 feet long, and 10 feet high – was sewn by Patty using 30-weight Agribon row cover and included a 110” sleeping bag zipper to allow easy entry. Diagonal guy wires and PVC clamps ensured the tent would not slip or be pulled due to winds. The bottom edge of the tent was weighted down. The tent was erected 4 weeks prior to the first harvest. The cost of the tent: approximately $200.
 
Traps: One SWD trap was placed outside the tent, and one trap inside the tent to monitor activity. A few male SWDs (with distinctive wing spots) were found in the outside trap during the season, but an exact number of SWD males was not determined, as there were many similar small insects in the trap. No SWD males were caught inside the tent.
 
Temperature observations: Because we were concerned about heat build-up in the tent, two recording thermometers were placed in the canopy of the cherry tree at 6 feet height under the Agribon from May 14 through June 19. Two additional thermometers were placed at the same height in an uncovered fig tree of similar size.
The low (nighttime) temperatures were on average 3 degrees cooler in the tree under the Agribon than in the uncovered tree (see graph). The high (daytime) temperatures under the Agribon were on average 2 degrees warmer than the uncovered tree. On days over 90F, temperatures under the Agribon were still only 2.5 degrees warmer, but during one 3-day period, temperatures averaged 8 degrees warmer.
 
Harvest: While harvest was a bit challenging (quickly entering/exiting the tent as well as moving ladders during harvest in a cramped space), overall it was a success with a full crop of undamaged, delicious cherries!
 
Other issues/drawbacks: The size of the tent required a number of us, with ladders, to assist in its assembly. The tent did suffer some damage near the bottom – we believe from a raccoon or similar creature. Several tears appeared overnight but were quickly repaired. Additionally, the tent suffered some wind damage along the bottom edge. We originally used 2’x4’s to secure the tent bottom, switching to soil to prevent further tears. On the plus side, we were able to disassemble the tent relatively quickly and store the parts for next season.
 
Conclusion: While our experiment was certainly not scientific, we do think of it as successful in that we avoided fruit damage of any kind on a fairly large tree with varying harvest periods. It does provide another option for homeowners who may be dealing with SWD. Although expensive initially, the cover should be reusable for possibly several years. In January 2013, we will espalier a cherry tree, and when the tree begins bearing we will secure row cover over it in the spring. This will provide demonstrations of both training and pruning on a trellis and a practical method of controlling SWD, since covering an espalier tree should be much cheaper and easier than a large tree.
—Tracy Lesperance, Cathy Coulter, and Chuck Ingels


For more information about the Sacramento County Master Gardener Program, including links to their wonderful 2013 Gardening Guide and Calendar, click here.

Peach Leaf Curl Returns in 2012

Another cool, wet spring has arrived in Northern California... 


...And with it, the return of peach leaf curl.

 Peach leaf curl causes leaves of peaches and nectarines to discolor, thicken, pucker, curl, distort and eventually fall off. The fungus overwinters in these trees as spores, usually in the new buds. The rains of late winter and early spring - or in the case of 2012, the above average rainfall of March (and now, April!) - splashed these spores onto the emerging leaves, causing more problems. Emerging shoots can die; fruit production can be reduced in severe infestations. Only rarely do reddish, wrinkled areas develop on fruit surfaces; later in the season these infected areas become corky and tend to crack.
 
         The good news is that a second set of leaves soon emerges and can develop normally when the rains cease and daytime temperatures steadily reach into the 80's. The bad news: too many years in a row (perhaps 3) of a serious peach leaf curl infestation can kill peach and nectarine trees.

 


 Studies at UC Davis have shown that nipping off infected leaves of peach and nectarine trees doesn't do much good this time of year.
 
         






If your peach and nectarine trees are showing signs of peach leaf curl now, the best thing you can do is to assist those trees through this stressful period. 

• Rake up any fallen leaves and pull weeds that are growing beneath the drip line of the trees.

• Fertilize the area thoroughly, if you haven't yet done so. 

• Before the weather heats up into the 90's, spread four inches of fresh organic mulch beneath those fruit trees. Organic mulches, such as compost, shredded branches or the fallen leaves of healthy shrubs and trees will help conserve soil moisture, hold down weeds and add nutrients to the soil as that mulch breaks down.

• If a tree is severely affected with peach leaf curl, stunted growth may result; consider thinning fruit later in the season. 

• Pruning in fall prior to applying any fungicides can reduce spore numbers overwintering on the tree and reduce the amount of fungicide needed. 

• If leaf curl symptoms occurred on your trees in spring, be sure to treat the following fall and/or winter to prevent more serious losses the following year.
 
         The experts at UC Davis advise pruning infected peach trees in the fall before spraying with a copper ammonium complex product with 1% horticultural spray oil added to the mix.

    In the good old days of fruit tree sprays (2009), 50% copper concentrates were the recommended course of action. Not any more. Copper sprays, such as Liquicop available currently are weaker (about 8% concentration). Lime sulfur has been removed from the market. Bordeaux mixtures are expensive and wasteful...and potentially caustic.

And to add insult to injury: recent tests conducted this winter at the Fair Oaks Horticulture Center by Chuck Ingels, UC Farm Advisor, show that the older, stronger copper sprays that are no longer available (Microcop, for example), along with lime sulfur, did provide the best control.

This recent trial involved treating (or not treating) individual branches on 10 different peach and nectarine trees

Still, that study did provide some good news about the future of peach leaf curl control:

The future of peach leaf curl control?

"Compared to untreated branches, Liquicop-treated branches averaged about 70% control," says Ingels. "Copper soap was slightly better at 80% control, Agribon (row cover material)  by itself was less effective at just under 60% control, and both Agribon + Liquicop and lime sulfur (late fall) followed by Microcop (late winter) resulted in nearly complete control.  Maxicrop (sea kelp) did not work at all and seemed to increase the severity on some of the branches."



The UC Davis Integrated Pest Management information on controlling peach leaf curl says, "Fixed copper products include tribasic or basic copper sulfate, cupric hydroxide, and copper oxychloride sulfate (C-O-C-S), but currently only liquid products containing copper ammonium complex products with 8% MCE (e.g., Kop R Spray Concentrate [Lilly Miller brands] and Liqui-Cop [Monterey Lawn and Garden]) are available to consumers. The most effective copper product, 90% tribasic copper sulfate with a 50% MCE (Microcop) is no longer available to retail outlets, because the manufacturer withdrew the product in 2010, although remaining supplies still can be sold."  

One of the reasons for that removal: repeated annual use of copper products over many seasons can result in a buildup of copper in the soil, which eventually can become toxic to soil organisms, and if it moves into waterways, can harm some aquatic species.


The removal of lime sulfur products was prompted by a rash of self-inflicted deaths in Japan in 2008 called "Detergent Suicides", which has since spread to the United States.


Bordeaux mixtures, a combination of copper sulfate, hydrated lime and water, are effective in controlling peach leaf curl, but come with their own set of warnings. According to the UC IPM Guideline entitled "Bordeaux Mixture":  "When applying Bordeaux, be sure to wear protective clothing, including goggles, because the spray deposit is corrosive, can permanently stain clothing, and is difficult to wash off." They also recommend wearing a dust and mist-filtering respirator when mixing in the hydrated lime. And that mixture can discolor anything it touches, including buildings and fences.


Although you can purchase pre-packaged Bordeaux Mixtures, they are not as effective as the mixture made from the individual components, reports that UC IPM Guideline. And that brings up the cost and waste involved: copper sulfate and hydrated lime are usually sold in large quantities, much more than the average homeowner needs for the backyard peach and nectarine trees. Storage involves mixing the leftover individual ingredients separately in water and storing in their own sealed jars. That UC IPM Bordeaux Mixture Guideline warns: "Be sure to clearly label both stock solutions and store them where children can’t get into them, since these materials, especially the copper sulfate, are very toxic and corrosive."

The synthetic fungicide chlorothalonil is the only non-copper fungicide available for managing peach leaf curl in the backyard orchard. Although one fall application may help prevent a spring outbreak of peach leaf curl, a second application in January or February, as the buds begin to swell, can be beneficial, as well.

But be sure to read and follow all label directions if you choose to use chlorothalonil, including this: 
"This product is toxic to aquatic invertebrates and wildlife. Do not apply directly to water or to areas where surface water is present or to intertidal areas below the mean high water mark. Drift and runoff from treated areas may be hazardous to aquatic organisms in neighboring areas."


Or this:  "May be fatal if inhaled. Harmful if swallowed or absorbed through skin. Causes moderate eye irritation. Avoid contact with eyes, skin or clothing. Do not breathe spray mist.


Or this: "This product contains chlorothalonil which is a
chemical known to the State of California to cause cancer."

No matter which spray method you choose, several days of dry weather must follow for the products to work. And that was the frustrating part of the winter of 2011-2012: the weather was comparatively dry during December, January and February...perfect for spray applications. The wet March (and now April) was the perfect vector for peach leaf curl, splashing spores of the disease to those branches that were unsprayed or incompletely sprayed.
 
         There are peach varieties that are more resistant to peach leaf curl. The downside: they may not be as flavorful as you might like. Peach varieties reported to be more leaf curl resistant include Frost, Indian Free, Q-1-8 and Muir; among nectarines, only the Kreibich variety is resistant, says UC Davis.

And for those who want to provide a helping hand next fall and winter to their suffering peach and nectarine trees: there's always spraying Liquicop combined with a spreader-sticker, followed by covering the trees with a row cover such as Agribon or other row cover fabric during rainy weather. Be sure to remove any covers during sunny weather to avoid overheating problems.

And it might not hurt to cover your peach trees during a rainy April, too.