- Author: Lynn Wunderlich
Hi Fodder Family! What a busy spring, heh? For those at the trough who are of the grape grower kind, this spring has brought questions concerning what the heck is going on with powdery mildew. To assist vineyard managers, I took a look at each of the 5 foothill mildew station's data and discuss it in a paper you will find linked here.
Recent rains, followed by fluctuating cool and warm temperatures, have got some managers spraying while others are waiting anxiously. Powdery mildew, considered by many the most severe disease of grapes grown worldwide, requires attention every year. The amount of management for any given season depends on a number of factors:
1.) Previous season's disease incidence in a particular block. The amount of overwintering mildew is proportional to the amount of disease in a given block the previous year. If you had mildew last year, carefully monitor that block and consider an early treatment.
2.) Grapevine growth-all green tissue (leaves, shoots) is susceptible to powdery mildew; and berries, only once they reach 8% sugar, are no longer susceptible to disease which works out nicely for harvest. Since all berries in a cluster aren't the same sugar content, consider 12-13% Brix a better number to use when ceasing treatment.
3.) Spring rain and temperatures. Ascospores (sexual spore stage) held in overwintering chasmothecia on trunks and cordons are the first mildew spores released in spring. Ascospores are released at temperatures from 50°-80.5°F (optimum is 68°-75°F) after periods of leaf wetness due to rainfall, heavy fog, dew or overhead sprinklers- about 2mm or 0.08 in. is required for spore release. Whether mildew growth from those spores occurs will depend on the presence of susceptible (green) grape tissue and subsequent temperatures. If temperatures are mild, the mildew fungus will "wait" until it warms up to grow and then produce more spores.
4.) In-season temperatures, regardless of leaf wetness, drive mildew growth the rest of the season. After initial ascospore infection, mildew reproduces via conidia (asexual spores) that thrive at mild temperatures, about 70-85°F. A risk index (RI) model using the in-canopy temperatures collected by the foothill stations calculates the predicted mildew disease pressure based on temperature. The RI for the six foothill stations can be found at this UCIPM link.
5.) Grape variety. Chardonnay, Rousanne, Carignane, and Chenin blanc are all considered highly susceptible to powdery mildew and should be monitored closely. Managers should note which of their varieties have had mildew problems in the past and give those special attention.
6.) Choice of fungicide. Once the RI is started for the season, the spray interval will depend both on the temperatures and subsequent risk index, and the choice of fungicide used. Fungicides should be rotated to prevent resistance developing in the mildew fungus.
For more information on powdery mildew, join us on May 18 at the Amador Fairgrounds for Foothill Grape Day. Doug Gubler, our UCCE grape plant pathology specialist, will be speaking on the highlights of his 33+year career. Doug is retiring soon, so don't miss the chance to hear him speak!
- Author: Lynn Wunderlich
Routine irrigation system maintenance, including flushing, can vastly improve system performance. That's what I learned last week when I met Brian Hockett, North West Kern County RCD, and Daniele Zaccaria, UCCE Irrigation Specialist, for a distribution uniformity check of our trial vineyard irrigation system.
Flushing of main lines, submains and laterals should be done while ensuring enough flow to flush out sediments (open only a few laterals at the same time) and continued until clean water flows out of the lines. How often should this be done? Most growers I've talked to said they do this once a year, but the amount of flushing required depends on your water quality-every few weeks may be needed to prevent problems building up at the emitter.
The quality of the water flushed will tell you what sort of problem you have. For example, precipitates forming on the emitters such as calcium carbonate may be an indication that acid injection is needed to keep the lines and emitters clear. Sulfuric acid or muriatic acid is used, it needs to stay in the system line for a period of time to work (read the labels for this information). Chlorine injections can work well
Distribution uniformity (DU) is measured by collecting the flow from emitters throughout your field and finding the average flow rate. Then the lowest 25% of the emitters measured are averaged, and this number is divided by the overall average.
DU= Average flow (lowest 25%)/Overall Average Flow. A DU of 90% or greater is considered "excellent"; 85-89% "very good"; 80-84% "good"; 75-79% "fair" and below 75% distribution uniformity is considered "poor". Growers should shoot for at least a "good" (80% DU or above) rating in order to be more efficient with the water they use. Afterall, you want to make sure the water you're pumping from the ground is actually getting to your crop, right?
Do you have an irrigation system problem? Do you know how much water you actually
Until next time...
- Author: Lynn Wunderlich
The most important thing to consider NOW about your cover crop is managing it in a timely manner-either by disking it in or mowing it down (made difficult with wet soils, I know)-as soon as possible.
Why the rush? You might ask. When budbreak occurs in our trees and vines, it signals the time when our crops become
The solution? Disk in or mow down that cover once you have budbreak in your crop...which has already occurred in our tree crops and is happening right about now in many grape varieties. If you've planned for what you want out of your
If you've chosen a grassy cover crop then you will likely be mowing it down as soon as the soil dries down enough to get in and do tractor work.
See my website handouts from my cover crop meeting for more information on choosing and managing a cover crop.
- Author: Lynn Wunderlich
1. Straw mulch. Straw mulch is a relatively simple way to hold your precious soil in place. Rice straw is particularly desirable because it has fewer weed seeds, but it is also high in silica which doesn't break down quickly and can actually tie up nitrogen. To remedy this, applications of nitrogen (no more than 1500 lbs. per acre, applied AFTER the rainy season) are recommended.
- Author: Lynn Wunderlich
Hello Fodder Followers! My "Bahder Discovers Red Blotch Vector!" post has received over 1600 hits to date-a testament to how critical this disease and research finding is to the grape industry.
First, just in case I need to (do I?), let me say this: no one in science works in a vacuum. Research is ALWAYS a collaborative effort, and every person involved, from grower-collaborator to first author, plays a role. And sometimes, Extension works! and plays a small but critical role in scientific inquiry.
So it started as it often does for us UC Cooperative Extension Farm Advisors: a bag arrives with a grower name, a phone number, and a short message "Can you help me identify this problem?"
In this case the route was circuitous: the grower attended a meeting where I made a Red Blotch presentation. I have worked with Sudhi Sudarshana (USDA virologist) since 2013, collecting photos of the disease on many grape varieties grown in the foothills. As most of you know, over 50 grape varieties are grown here, in a range of soils and
My presentation made this grower think about some red leaf symptoms in his own vineyard. He had been told that the red leaf was due to potassium (K) deficiency and so he had made applications of K for the past couple of years. Yet, the red leaf continued and appeared to be spreading. So off to the UCCE office he went to drop off a bag of red leaves with a note.
Certainly Brian would likely have made his findings without this site: the team was already hot on the trail before I came along. I did not know about Brian's transmission test results before the webinar, though I knew he had narrowed the vector candidates. So when I watched his presentation I felt pride and excitement for him and for the team.
Diagnosing problems is a critical and sometimes undervalued role for us UCCE Farm Advisors. We are the "boots on the ground" for our colleagues on campus and most of us love this aspect of our work. But few realize the number of hours that can go into a single farm call. Most often the reward is a grower's gratitude (which is enough!), and sometimes, it's a little bit more.