Vine Covers for Table Grapes

Table grapes are a labor and material intensive crop; annual operating expenses may exceed $7,000 per acre. Thus, it is critically important for growers to protect late season grapes from rain, as exposure to precipitation within six weeks of harvest can stimulate the development of rots and molds that may render them unmarketable. Rain damage may be avoided by harvesting the fruit before the fall rainy season, but the grapes of some late maturing varieties may not ripen by then, and the price paid for grapes often increases towards the end of the season, thus providing an incentive to harvest fruit as late as possible.
Thus, many growers with late-season table grapes cover their vines with sheets of plastic film, beginning in late August or early September. At least two different colors of film, green and white, are
available, but data to distinguish the potential differences the two films might have on vine physiology or fruit quality are not available.  The cost to procure, install, and remove the films is very expensive, so growers need objective information on film performance.  To provide such information, we have just begun a study to determine how the two most common film covers may affect canopy microclimate, vine physiology, and fruit quality, to help optimize the productivity and quality of late-season table grapes in California.  Follow the blog and we'll update you periodically on our findings.

 

Different vine covers being evaluated in a commercial vineyard near Fresno, CA.

Soon after installing the covers, noticeable changes in canopy microclimate were observed, including higher temperatures, and condensation was evident on the inside of the covers.

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.