Handling Raisins in the Event of Rain

By all accounts, grapes are maturing later than usual this year.  As a result, many growers have delayed harvest to enable the berries to accumulate more sugar.  The late season has also prompted some labor crews to seek other work opportunities and thus some other growers are harvesting later than they would like due to difficulty in securing labor.  In any case, a late harvest increases the risk that rain could occur while the raisins are drying. 
The potential for rain to damage raisins on trays depends on the amount of rain, and the weather conditions following a storm.  A light rain storm, dropping 1/4 inch or less of precipitation, will probably cause little if any damage, especially if followed by dry weather.  A moderate storm, producing 1/4 inch to 3/4 inch of rain could cause minor damage, if followed by ideal drying weather, or substantial damage, if followed by cloudy weather.  Rainfall over 3/4 of an inch will almost certainly cause extensive damage, regardless of the weather that follows. 

Severe storms may flood vineyards, leading to complete crop loss.

Fungal infections led to complete crop loss following severe rains.

If a manageable amount of rain occurs during drying, growers should keep in mind certain handling methods and finish-drying options that will help minimize crop damage. 

First, "slip" the trays, to prevent them from sticking to the soil and wicking moisture from the soil into the tray.  This can be achieved by sliding the trays just a few inches--enough to break the seal between the soil surface and the paper trays.

Next, invert the fruit on the trays as soon as possible to expose "bottom" fruit to sun and air.

A raisin tray, immediately after inverting the partially dried fruit.  The green-colored berries were not previously exposed to the sun, which is why they have not yet browned.

Do not try to save any fruit that is stuck to the bottom of the trays, nor allow such fruit to be mixed in with the other fruit.  If necessary, replace the trays with new ones, or invert the trays (if the bottom side is clean), leaving stuck fruit on the underside of trays, and placing unstuck fruit on the "clean" (formerly bottom) side of the trays.

Pick moldy or rotten fruit from the tray before rolling, and remove rolled fruit from the field as soon as possible.  If moisture content is >18 percent, do not fill bins more than half-way.  Begin on-farm or commercial drying as soon as possible--immediately if moisture is >22 percent.

Breeding New Pest Resistant Rootstocks for Grapes

With bloom well underway, grapevine breeders and geneticists, such as Peter Cousins, with the USDA’s Grape Genetics Research Unit, in Geneva, NY, are busy making crosses between pairs of vines having favorable traits.  Cousins is a rootstock breeder, so traits that interest him include resistance to root-knot nematodes, soil-borne pests whose feeding activities can cause extensive damage to grapevine roots, thereby weakening the vines and reducing yields.  Though Cousins, is based in NY, he is cultivating some advanced selections from his rootstock breeding program in a vineyard at the Kearney Agricultural Research and Extension Center in Parlier, California.  By growing the vines at Kearney, Cousins can assess their performance in the San Joaquin Valley, one of the most important grape-growing regions in the world.  Vines that maintain their nematode resistance, grow well, and are easily propagated, may eventually be released as commercial rootstocks, and serve as a genetic resource for future crosses aimed at further strengthening their nematode resistance by incorporating as many resistance mechanisms for resistance as possible.

Some wild grapevine species have unique nematode resistance mechanisms, and are thus used as parents in rootstock breeding.  Cousin’s goal is to incorporate as many different forms of resistance as possible into new rootstocks, so some advanced selections are complex hybrids of several species.  Wild grapevines, and their hybrids, are dioecious; invididual vines bear clusters of either male or female flowers.  Dioecious vines are relatively easy to cross as the flowers do not have to be emasculated; to prevent unwanted crosses, he simply encloses female flower clusters in paper bags prior to bloom. 

As bloom approaches, he periodically he taps on the bags, listening for the rattle of detached petals that signals the blossoms are ready to be pollinated.  Properly prepared, pollen, which Cousins prepares by grinding flowers in a coffee mill, can be stored for a relatively long time, facilitating crosses between vines that might not flower at the same time.  To pollinate, the bags are removed, the appropriate pollen is slathered on with paint brush, and the cluster is labeled, and recovered with the bag.  Cousins will return in the summer, to extract the seeds from the fruit.  Seedlings will then be screened for nematode resistance, and the best selections may eventually end up returning to Parlier, to be evaluated alongside their parents.  To learn more about Dr. Cousin’s research program, and see his rootstock selection vineyard, plan to attend Grape Day at Kearney, August 18, 2011.

Prebloom zinc sprays can improve fruit set

The earliest grapevine varieties are starting to bloom at the Kearney Agricultural Research and Extension Center in Parlier, near Fresno, California.  Some of the grapevines here, as in other parts of the state, are susceptible to mild zinc deficiencies, the only obvious symptom of which may be reduced fruit set.  Varieties having 'Muscat of Alexandria' parentage are particularly prone to zinc deficiency, as are vines on sandy soils, or those grafted to rootstocks having Vitis champinii parentage, including 'Freedom' and 'Harmony'.   All vines at the station are subject to an annual zinc foliar spray during a two-week pre-bloom window that closes when 60 to 70% of flowers have opened.  This year's treatment is scheduled for Monday, May 9. 
Neutral zinc is the most effective product.  Two to three pounds of neutral zinc per acre are suggested to correct moderate deficiencies, though lower rates may be acceptable for minor deficiencies.  Dilute sprays are most effective, using 100 to 150 gallons of water per acre. For more information on the diagnosis and correction of zinc or other micro or macro nutrient deficiencies, review L. Peter Christensen's excellent book chapter Mineral Nutrition and Fertilization.