Let it Rain!

Dr. Terry Bates

8/3/01 -

I do not need to tell most of you that we are experiencing a bit of dry weather this summer. The question is: How are the grapevines responding to this dry weather?

At the Vineyard Lab in Fredonia, we measure soil moisture, leaf water potential, and leaf photosynthetic rate to monitor how the vines are responding to the dry weather conditions. For leaf water potential we use a device called a pressure bomb to measure vine water status. For this measurement, a leaf is cut from the vine and placed in a sealed chamber and the air pressure is increased in the chamber until water is seen coming out of the petiole. In this case, you have to think of the leaf as a sponge. When you squeeze a sponge that is saturated with water, it takes very little pressure to get water to drip from the sponge. It takes more pressure to get water to drip out of a damp sponge and a lot more pressure to get water from a dry sponge. The pressure bomb does the same thing to a grape leaf. A vine with good water status has low pressure bomb readings (it takes low pressure to squeeze out the water). A vine under water stress has higher pressure bomb readings.

So what is the pressure bomb telling us? Around July 16th, our pressure bomb readings were around 6-7, indicating no water stress. Around July 23rd, pressure bomb readings were in the 8's, indicating that the vines were just starting to experience some water stress. On July 31 and Aug 1, the readings were between 9 and 11, indicating water problems and reduced leaf photosynthesis. The good news is that with all of our dry weather, the vines are just recently showing signs of reduced photosynthesis. The bad news is that if we do not get some significant rain soon, lower photosynthesis will translate into lower vine production - most likely in the form of reduced vine size or vine capacity for next year.

At the lab, we have different viticulture treatments that deal with conserving soil moisture (weed control), improving soil moisture (irrigation), or improving the root systems' ability to get water (rootstocks, good soil fertility).

This figure shows leaf photosynthesis on Aug 1 on some of the different treatments at the Fredonia lab. Starting at the left, mature Concord vines grafted onto C3309 rootstock, either under cultivation or sod row middles, have good photosynthetic rates. C3309 roots are phylloxera resistant and very deep, which helps the vines maintain good water relations during dry years. The problem with grafted vines is that they can be over-vigorous in wet years and they require slightly different management than own rooted vines. Moving to the right, irrigating own rooted vines also helps maintain good water relations during dry weather. Vines with a lot of leaf area and large crops, such as high bud number vines, are more susceptible to water stress and require more irrigation than smaller, moderately cropped vines. Bars 4 and 5 look at water conservation through weed control, the most common water management tool in the belt. Own rooted vines with good chemical or mechanical weed control at bloom are starting to show more water stress with photosynthesis reduced by about 25% from the maximum potential photosynthesis. Own rooted vines with poor weed control (own sod) have photosynthesis reduced by almost 50%. Young (3 year old) vines are under even more water stress because their root systems are more shallow and less extensive than mature vines. In addition, young vines that have poor mineral nutrition because of low soil pH have even more restricted roots, higher water stress, and very low leaf photosynthesis.

Water availability and root function are starting to affect vine photosynthesis this season and will have an impact on vine capacity for next season. For most of you that have own rooted, un-irrigated vines that used weed control at bloom, the best thing to do is to pray for rain. Additional weed control at this time is a risky option. There is little soil moisture left to conserve so additional weed control might not do anything for you; however, we could argue that any rain that we do get could be made available to the vines if the weeds were absent. We have seen increased vine size with a second row middle herbicide applications in dry years like '98 and '99. However, those years were wet early and dry late so the second herbicide application conserved the water that was there. This year is dry early and soil moisture is already low. The decision for additional weed control must be weighed against cost, the low crop situation, and the possibility that the rain could turn on and turn a cover free vineyard into a sloppy mess at harvest.