What good is a bunch of cold grapes?

Well here we are, firmly in the middle of Summer and damn it’s Hot!  Hot everywhere but here in wine country that is.  I have heard that this is going to be the hottest year on record, McClatchy has a report stating that according to NASA 2010 is on track to be the hottest year ever, 2009 WAS the Second Warmest ever recorded according to NASA, but here in Wine Country we’re hoping to get some Summer weather to warm up these grapes.  By this time of year the grapes should be well into verasion.

Grapes in Verasion - Photo Credit Tablas Creek Vineyard Blog

I wrote up a piece earlier this year that talked a bit about how the long and late rains might affect the vineyards and I thought I’d follow it up with some info on what this cold might be doing to our beloved grapes.

Once again I’ve called upon the local experts to get the lowdown on some of the terms that I hear used and also asked for some commentary and color on their predictions of the 2010 growing season and vintages.

Here’s what Dr. Merilark Padgett-Johnson, instructor for the Viticulture program at the Santa Rosa Junior College.

NC: I was wondering if you could add some color to the discussion.  Is there an easy way to explain what “degree days” are, so non-farming/non-viticulturalists could easily grasp the concept, and include some of the science that goes into calculating it?

M P-J: Degree days are what it sounds like, heat units accumulated over time.  Grapevines achieve net growth when the temperature is over 50 degrees F.  Under that threshold, no net growth.

Degree days (DD) are calculated by taking the daily average temperature  (max temp + min temp /2),   then subtracting 50.  This is gives you the degree day figure for that day. For example, if the max temp for one day is 78, and the minimum is 56, that average is 67; subtract the threshold 50 from 67, you get 17 DD for that individual day. Degree days are calculated individually, then accumulated daily over the course of the growing season.  Different cultivars have different degree day requirements for ripening their fruit.

NC: Secondly I was wondering if you had any commentary on the weather and your predictions for the effects it may have on the 2010 vintage.

M P-J: Predictions I don’t do, who knows?   We just hope to get enough degree day accumulation from here on out, to meet the DD necessary to ripen the fruit.

I asked my sources at another local vineyard to pass my questions on to their Viticulturist and they were kind enough to respond and have provided some outstanding insight and detailed information about their concerns.  I’m also including a document written by them which they put together back in May with predictions on this year.  I’ve taken some basic Viticulture classes and this is some interesting stuff.  I’m very sure I don’t grasp all of the nuance, but any of you out there who are steeped in vineyard management and viticulture should really appreciate it!

NC: I was wondering if you could add some color to the discussion.  Is there an easy way to explain what “degree days” are, so non-farming/non-viticulturalists could easily grasp the concept, and include some of the science that goes into calculating it?

Viticulturalist: A degree day is the measure of accumulated heat between two set points (calendar dates).  It’s known that certain varieties of grapes need a set amount of degree days to ripen.  For example if you wanted to plant variety “A” in a valley, and you know it takes 2500 degree days to ripen variety “A”, you wouldn’t plant it in a valley where the total degree days are 1500.  So, in regards to knowing what to plant and where, it is a very useful tool.

The degree day model is also very good for predicting the life cycles of insects and plant pathogens.  This is extremely helpful because we can time our spray application to be the most effective.  There is a degree day calculator available for most plants, insects, and plant pathogens and is readily available.

We know are fields very well, and we know the timing of three major events, bud break, bloom, and verasion.  We can gauge whether the season will be early or late based on the timing of those three events.  My experience has been farmers don’t talk about degree days in regards to ripening and when to harvest.  Grapes samples are taken for analysis two to three times a week for at least a month before harvest.  The analysis will basically give the percent sugar, titratable acidity, and pH.  Those are what usually determine when to harvest.

NC: Secondly I was wondering if you had any commentary on the weather and your predictions for the effects it may have on the 2010 vintage.

Viticulturalist: I’ve attached a document my assistant, <name deleted> and I wrote about the upcoming 2010 season back in May.  It has turned out a very good predictor of what we are seeing in the vineyards in terms of bloom/shatter in relation to yields, and Botrytis Bunch Rot.

My prediction for the upcoming vintage is the slow, cool, wet, growing season may not be a bad thing for the early ripening varieties like Chardonnay, Pinot Noir, and Sauvignon Blanc.  Provided they are planted in an early ripening area.  The grapes that manage to ripen without getting a bunch rot infection will be outstanding.  I just don’t think there will be many grapes without rot this year.  I also think some of the later varieties like Cabernet Sauvignon, Merlot, Syrah, and Zinfandel may never make it off the vine.  Those later varieties will have trouble making sugar (ripening) because of the cool, extended growing season.  The possibility of them rotting on the vine is very high.

NC: Do you have any other thoughts on how strange the year seems?  For example all the rain we had, well into late Spring/early Summer, etc.

Viticulturalist: The running joke in the vineyards is no year is ever normal.  However, this year is particularly challenging.  The long wet cool spring, followed by a cool and summer has placed the vines about 3 weeks behind schedule.  In a “normal” year we would begin harvest the last week of August.  We are expecting harvest to begin around the 10th-15th September.  The longer the grapes are on the vine the greater the risk for pathogen damage, particularly bunch rot.  It is quite possible we could lose at least 20% of the crop.

Here is the growing report shared with me detailing predictions of the 2010 Grape Growing Season:

The 2010 growing season has been plagued by nearly two solid months (April and May) of below normal temperatures and above normal precipitation.  In fact, statewide, the month of April ended up being the 12th coldest on record.   Furthermore, the excessive precipitation delivered in April made it the top ten wettest in the North Coast in the last 100 years of record keeping.  Currently, for the month of May, we are experiencing unseasonably cool temperatures and weekly precipitation events.

This combination is contradictory to what is commonly considered ideal viticultural conditions for these two critical months.  The consequences of the prolonged cool and wet spring have not yet revealed themselves completely, but from experience, it is possible to predict with some confidence what they will be.  Two areas in particular are worth noting: an increase in disease pressure both short and long term and the unavoidable effect on bloom and eventual yield.

The primary disease concern is Botrytis cinerea, a fungus favored by wet conditions.  The resulting infections can affect shoots, leaves and clusters with necrosis that often leads to the loss of the affected tissue.  Only the much cooler than normal temperatures have prevented widespread infections from appearing this year.  The major concern is the ability of the fungus to go dormant inside the flower cluster, and when warmer and drier weather returns and the ripening clusters are exposed to free water of any kind – be it from rain, fog or heavy dew, the fungus will begin to sporulate and the cluster will begin to rot from the inside.  Chardonnay and Pinot Noir are well-known for their susceptibility to the fungus, and it should be expected that botrytis bunch rot will pose a serious threat as the season moves into harvest.

We have also noted the presence of a rarely-seen abiotic complex this year that is affecting Pinot Noir.   The complex is characterized by the sudden discoloration and death of leaves located at the mid-shoot position.  This affliction has been observed by other growers in our area and appears to be quite pervasive.   It has recently been determined that the necrosis is caused by an accumulation of ammonia resulting from an abnormal build-up of nitrogen.  This occurs because the unseasonable cold has retarded vine growth, and the nitrogen which has been taken up by the root system is not used but instead pools up in the shoots and leaves.  It is currently unclear what the progression of this problem will be or what longer term effects it will have on the affected vines.  However, with soil temperatures running in the 50’s, (another unfortunate consequence of the cold, damp spring ) – as opposed to the 70’s, where they would be in a normal year, it is almost certain this problem will continue to appear as long as the late-season rain and abnormally cool regime prevail.   We have also seen many mid-cane shoots in Chardonnay which have slowed their growth to the point of cessation, with the shoot tips giving every indication their growth has stopped for the year.  This is problematic because the shoots have not reached a length which will adequately ripen the grape clusters which they are supporting.  While it is hoped this phenomenon may be reversed somewhat by a return of normal late spring/early summer temperatures, it may be difficult to achieve given the stage of the vine growth cycle.

With regard to the inhibition of bloom, we have already observed many of this year’s early-blooming grape flowers being prevented from successfully completing the entire process because of the excessively cool and damp weather.   The cap (calyptra), which covers the male flower parts (the filaments and the pollen-bearing anthers), is staying attached to the flower instead of falling off.  This keeps the pollen grains from landing on the opening of the female flower part (the stigma) where a pollen tube will begin to grow leading to successful fertilization of the ovary and the eventual production of a grape berry.  The dampness also has a deleterious effect on the ability of the pollen grains to be easily released by the anthers that hold them which also has a negative effect on pollination.  For these reasons, viticulturists are fond of seeing bloom time weather that is warm (not hot – lest the pollen grains become dessicated in the heat) and dry.  In due time, the amount of unfertilized berries will shatter off of the grape cluster, and we will have a much better idea of how our potential yields have been affected by this almost unprecedented combination of rain and cold.

There are very possibly two other negative consequences of the late and protracted bloom: irregular ripening of the fruit and a later than normal harvest which may confront October rains should they occur.  The irregularity of the bloom and pollination will likely end up causing problems for accurate sampling as, even within a single cluster, flowers may have been pollinated serially over a period of several weeks.  Under these circumstances, berry sampling would be an exercise in frustration.  This is perhaps a year when cluster sampling might prove to be a considerably more accurate tool in assessing fruit maturity.  If the maturity of normally earlier harvested vineyards is pushed back to later in the year, while the usually later harvested blocks remain closer to their average harvest date, it is possible that many disparate blocks could be ready for picking at the same time – a logistical nightmare for both the viticultural and enological sides of our operation.

One last point to make would be the effect on the 2011 crop.  In all likelihood, there will also be some longer term effects resulting from this spring’s egregious weather.  It is well-known that bud fruitfulness, which heavily influences the potential size of a crop, is determined in the late spring and summer prior to that crop’s actual appearance on the vine.  A basic requirement of fruitful buds is ample sunshine on the shoots and canes that bear them.  Given the lack of sunlight so far this year, bud fruitfulness (or lack thereof) for next year’s crop is rapidly becoming an issue.  In addition, the excessive amount of water in the ground will only encourage rank vegetative growth on the vines – particularly when the weather gets warmer – leading to increased shading of shoots and canes and necessitating leaf and lateral pulling to encourage better sunlight penetration into the canopy.  If, as we suspect, a number of the shoots have stopped growing as mentioned above, the probability of this abundant supply of water being channeled into lateral growth becomes even higher, exacerbating the shading of next year’s fruiting buds and leading to a lighter yield in 2011.

As you can see, this is going to be an exceptionally strange year, even the experts are expressing concern and there may even be lingering effects for the following vintage!  I hope this is interesting to you too.  I’ll try to follow up with these folks after harvest and see how everything turned out.  So keep checking back!