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K. Peterson & M. Krasnow
A note for our readers who may not be well versed in the movie The Princess Bride. First off, Inconceivable! Second off, writing this blog and the connection between R.O.U.S.s as well as other quotes from the movie, brought us so much joy. It is a classic film that we highly recommend you check out, or re-watch.
Many vineyards are looking into ways to improve vineyard efficiency and cut costs while not compromising quality. As vineyard managers and viticulturists, this process is largely swayed by perceived risk vs input cost as compared to the cost of potential negative outcomes. One of the top 3 offenders in this battle of perceived risk are what from now on we will think of as R.O.U.S.s – Rounds Of Unnecessary Spraying.
Rounds of Unnecessary Sprays? (“Rodents of Unusual Size? I don’t think they exist.” – Westley)
Spray programs can either help make or quickly break fruit quality in the vineyard. So naturally the idea that it is better to start canopy spray programs “early,” regardless of disease pressure (current and past season), than miss the boat and end up fighting a losing battle all season is common practice globally. It’s this mentality that contributes millions of dollars of inputs, in addition to adding to the overall global carbon and chemical footprint of the wine industry, yet assessment of this practice is not discussed enough.
Given the current state of the global wine market, and the increasing costs of farming such that growers are looking for opportunities to improve operational efficiency (cut costs), we reckon it’s not a bad time to climb on this soap box and share our thoughts and experiences on what many deem “not worth the risk.”
Clearly not all growing regions are the same, and neither are the mixture of pests, diseases, agricultural and sustainability program regulations, etc., and we’re not pointing fingers and saying R.O.U.S.s are found in every vineyard every season. Rather we’re encouraging thoughtful consideration around when to start spraying, with some data which may potentially remove some of the perceived risks around early season sprays.
Powdery Mildew – “Let me explain. No, there is too much. Let me sum up.” Inigo Montoya
Powdery mildew (Erysiphe necator) is a key pathogen of grapes in all regions because of its ability to survive relatively dry conditions that are not conducive for the growth and reproduction of other fungal pathogens. All green succulent tissues of grapes are capable of being infected by powdery mildew, but for infection to occur spores must germinate and surface mycelial growth needs to find an entry point into the tissue, like stomata on leaves. Optimal temperatures for spore release and germination are 20-25 C, and under these conditions release, germination, and infection can happen within 12 hours if leaves remain wet. However, all these processes are reduced at temperatures under 20 C and above 25 C. These temperatures play into forecasting risk tools, which are widely available in many growing regions.
As a comparison here are the graphs from NZ Winegrowers’ Vinefacts for Hawke’s Bay in 2023 and 2024. What you can clearly see is the Gubler Model used in these graphs reveals a couple of things:
Early season risk varies from season to season
The model does not show severe risk until mid-November in 2023, and late-October in 2024.
In this example, bud break across the region is typically occurring in mid to late-September, which for many (not all) vineyards globally is the start of canopy spraying. Tracking with standard practice, simple math suggests there could be anywhere from 2-5 spray rounds before conditions were suitable for powdery mildew spread that actually required regular control.
“It's a Trap” – Admiral Ackbar (Star Wars – we know, we know– but read the section before judging)
During my time in Dr. Skinkis’s Lab I had the privilege of not only working for her, but also with some great people at the USDA-ARS in Corvallis. Dr. Walt Mahaffee, “the powdery mildew” guy*, had a PhD student working on a trial across commercial and research vineyards using spore traps to identify presence of airborne powdery spores as a tool to initiate early season spray programs. I’ve linked one of the publications from this trail work at the end – but the takeaway from the paper, and experiencing this trail firsthand was that vineyards were spraying too early. The average inoculum-based spray program as reported for 2010-2011 was 3.3 fewer sprays compared to grower control, with no significant difference in powdery mildew between the two treatments.
Having farmed those seasons – 2011 in the Willamette Valley was a good season to test this out. A late season with a bumper crop, I remember people harvesting in November, which was wild. As the trail continued in later years, the only thing that really popped up which seemed to be linked to the delayed early season sprays were some vineyards seeing erineum (blister) mites crop up after a few years of reduced sprays. This was nothing that really impacted the crop, but raises the point that early season sprays can be controlling pests and diseases (as well as beneficials) other than powdery mildew. (Thiessen et al, 2016 - free access link to paper in references)
"Risk comes from not knowing what you’re doing" – Warren Buffet (again – not the princess bride, but too appropriate not to use)
There are a million and one ways to second guess yourself as a viticulturist and vineyard manager, not to mention a considerable amount of pressure around producing awesome fruit year after year regardless of what mother nature throws your way. Unfortunately, this can undermine implementation of sustainable practices, as far too much blame gets pushed on the vineyard when things don’t turn out quite right.
Don’t believe me – I once had winemakers ask what I did differently in 2 out of 12 Pinot blocks at a vineyard, because the ferments ended up ultra reductive. It was an organic vineyard, but sulphur was applied equally across all blocks, no difference in fert/cover crop regimes, tonnage was as requested, fruit was clean. So when I asked to see the ferment details for all the blocks in that vineyard, you know what stuck out – those exact 2 blocks were the only 2 blocks where they had done a 40+ day cold soak on the fruit. But to them Occam’s razor was this was a result of something I had done in the vineyard.
A viticulturist I respect once said, in relation to flying helicopters for frost protection (and again for spray programs) – paraphrased - No one is going to question you for putting up a helicopter based on your judgement of risk when they’ve already been flown in for the night. But if you didn’t put them up and you get frost damage they’d be knocking down your door.
In our experience, R.O.U.S.s occur because no one is going to ask why industry standard timing is calendar based, so why add another judgement call on to your plate? But here’s the thing, monitoring the Gubler Model to make an informed call on when to start your spray program, is less of a judgement call than when to start irrigating with an uncalibrated soil probe (see what we did there). If we have these tools at our disposal that are aimed to help inform decisions based on science, aren't these no longer technically judgement calls?
The chem alone for an early season spray across Marlborough is upwards of $600,000 (NZD). This cost does not take into account labour or tractor hours, nor does it take into account intangibles like driving on wet and compactible soils in spring. By being a bit bolder and more confident in the science, we can skip several sprays every year. To us this cost savings makes a lot more sense than trying to cut corners skimping on things like wire lifting, canopy management, and fertiliser.
Literature Cited
Thiessen, L.D., Keune, J.A., Neill, T.M., Turechek, W.W., Grove, G.G. and Mahaffee, W.F. (2016), Development of a grower-conducted inoculum detection assay for management of grape powdery mildew. Plant Pathol, 65: 238-249. https://doi.org/10.1111/ppa.12421
Thiessen, L. D., Neill, T. M., & Mahaffee, W. F. (2018). Interruption and reduction of Erysiphe necator chasmothecia development utilizing fungicidal oil. Plant Health Progress, 19(2), 153-155. https://apsjournals.apsnet.org/doi/pdf/10.1094/PHP-11-17-0070-RS
Gadoury, D. M., Pearson, R. C., Riegel, D. G., Seem, R. C., Becker, C. M., & Pscheidt, J. W. (1994). Reduction of powdery mildew and other diseases by over-the-trellis applications of lime sulfur to dormant grapevines. https://www.apsnet.org/publications/PlantDisease/BackIssues/Documents/1994Articles/PlantDisease78n01_83.pdf
Thiessen, L. D., Keune, J. A., Neill, T. M., Turechek, W. W., Grove, G. G., & Mahaffee, W. F. (2016). Development of a grower‐conducted inoculum detection assay for management of grape powdery mildew. Plant Pathology, 65(2), 238-249. https://bsppjournals.onlinelibrary.wiley.com/doi/full/10.1111/ppa.12421
Vinefacts information retrieved from nzwinegrowers.com
Other Helpful Resources
Bettiga, L. J. (Ed.). (2013). Grape pest management (Vol. 3343). UCANR Publications.
Grape powdery mildew model - UC Davis IPM. Accessed 20 November 2024. https://ipm.ucanr.edu/DISEASE/DATABASE/grapepowderymildew.html
For those who have the same appreciation for the Princess Bride as we do – we have put together a few quotes that we have translated into vineyard management principles.
"Don't rush me, sonny. You rush a miracle man, you get rotten miracles." - Miracle Max
Don’t rush your spray rounds – driving too fast reduces coverage even at the same application volume per area sprayed
"There's a big difference between mostly dead and all dead…. Now, mostly dead is slightly alive. Now, all dead ... well, with all dead, there's usually only one thing that you can do—search through their pockets for loose change."- Miracle Max
Lime sulphur while it can reduce inoculum, it by no means eliminates total risk, so if you have to spray to control what could not be sorted with that application, in my opinion you’re better off keeping tight intervals early season – if powdery got a bit out of control last season. Save the lime sulphur for mothballed vineyards coming back into production, or if there were other pathogens that you’re worried about. It’s hard on equipment, not nice to spray, and a good thing to avoid applying if you don’t have to.
"I just want you to feel you're doing well. I hate for people to die embarrassed." - Fezzik
This sums up what I like to call “for the winemaker sprays” – e.g. botryticides at harvest. Sure, it may sort some botrytis infections, but the inoculum is often inside the bunch (and berry skin) and those sprays “don’t mean what you think they mean
"Hear this now: I will always come for you." - Westley
Chasmothecia to the grape vines as they're washed on to the vines as the leaves fall off post harvest
"I always think that everything could be a trap, which is why I'm still alive." - Prince Humperdink
Chasmothecia and bud perennation are extremely effective evolutionary survival tactics of powdery mildew. But spraying early is not always the solution. If conditions are not right for spore release and germination early sprays may not reduce risk. Calendar based programs can equally over apply as they can under apply – in a hot spring with plenty of soil moisture shoot elongation between early season rounds can leave a significant amount of susceptible tissue not protected.
"Death cannot stop true love. All it can do is delay it for a while." - Westley
I’ve always said if you can’t find powdery in a vineyard, you’re not looking hard enough. Even with the best of control, you still have to spray vines every season, and the reality is powdery resistant cultivars do not necessarily mean zero sprays.
"Life is pain, Highness. Anyone who says differently is selling something." - The Masked Man
For chem reps you don’t trust – look to the science.
“You seem a decent fellow, I hate to kill you.” - Inigo Montoya
“You seem a decent fellow, I hate to die.” - The Masked Man
Sulphur sprays are such a blessing when it comes to PM control. They’re organic, relatively cheap, and low risk of PM resistance. However while sulphur may knock out powdery mildew as well as some pests like mites, it is equally lethal to some beneficial mites AND too many sprays will have winemakers at your door step blaming you for their reductive ferments. So while it is a “decent fellow” within the realm of spray programs, over/unnecessary use can have detrimental effects we must not forget.
"As you wish” - Westley
Reserved only for the good winemakers and chem reps you trust, or when the owners are visiting and staying on the vineyard.
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