Plasmopara viticola (a.k.a. Downy Mildew) is one of the most common diseases and more serious threats for Vitis vinifera varieties all over the world.¬† Early symptoms occur on the leaves of grape vines, exhibiting light green to yellow spots called ‚Äúoil spots‚ÄĚ (because they look greasy), which can be found on the lower surface of the leaf.¬† These spots are actually spores, which are spread through passing winds.¬† These spots eventually become necrotic and the tissue dies, which can severely weaken the plant and reduce its‚Äô winter hardiness.¬† Berries that are infected will turn a dull-green or reddish purple, and will fall from the cluster.¬† This effectively reduces crop yield.
Since cultivars that are resistant to P. viticola are extremely rare, it is essential to apply fungicides (particularly in rainy climates).¬† For organic vineyards, which are becoming increasingly popular all over the globe, downy mildew is controlled by regular sprays of copper compounds.¬† The use of this compound has been widespread, and for a very long period of time (~150 years).¬† Early excessive use of copper has resulted in the accumulation of copper in the topsoil of vineyards, which has had negative effects on the surrounding environments.¬† In Europe, there are now restrictions on the use of copper, allowing for no more than 3 to 6 kg/ha per year, depending upon which country in which the vineyard is located.¬† There is currently an interest in restricting these limits even more, which will require reevaluation of the use of copper to control downy mildew, and introduces a need to find other alternatives for the use of copper in controlling downy mildew that still fall under organic farming principles.
The current study presented today is the first (to the authors‚Äô knowledge) that provides a comprehensive evaluation of alternatives to the use of copper in controlling downy mildew, and examines a wide array of ingredients that are nontoxic and acceptable under organic farming rules.
Both indoor trials (artificial infections) and field trials (natural infections) were performed in this study.
112 substances were tested for effectiveness against downy mildew: 9 were from animal origins; 7 were biocontrol agents; 1 was a homeopathic preparation; 4 were microbial extracts; 8 were natural derivatives; 48 were plant extracts; 2 were physical methods; 6 were synthetic materials; and 6 were mixtures of the previous groups.
Indoor trials were conducted in greenhouses or growth chambers in Italy and Switzerland.¬† Experimental sets contained an untreated control (water), a standard treatment (copper hydroxide) and 8 to 12 of the test treatments.¬† The dosages used were given via the manufacturer‚Äôs recommendations.¬† All experiments in Italy had four replicate plants per treatment, and all experiments in Switzerland had six replicate plants per treatment.
Grafted cuttings of Pinot Gris (on Kober 5BB) and seedlings of Chasselas were grown in individual pots on peat-rich pre-fertilized soil.¬† Plants were kept under natural light at temperatures between 18o and 28oC.¬† Plants were used for experiments when they had 10-15 fully developed leaves on 1-3 shoots.
Sprays and Inoculations
All treatments were sprayed on the tops and bottoms of all leaves until near run-off.¬† The plants were then left to dry before being inoculated with downy mildew.¬† Downy mildew treatments were sprayed on the abaxial surface of the leaves, between 6 and 12 hours after the treatments were applied.¬† After inoculations, plants were incubated at 20-21oC and 80-99% relative humidity in the dark for 24 hours.¬† Then, plans were maintained at a temperature of 18-25oC and 60-80% relative humidity, and undergoing a 16 hour day and 8 hour night schedule.¬† Once ‚Äúoil spots‚ÄĚ were noted on the leaves (usually 6-10 days after downy mildew inoculation), plants were once again incubated overnight in the dark at 20oC and 80-99% relative humidity (to promote sporulation). ¬†
Disease incidence (percentage of leaves with oil spots or visible sporulation) and disease severity (percentage of the leaf area occupied by sporulating lesions) were calculated.
The best treatment(s) that displayed efficiencies comparable to copper applications under greenhouse conditions were used for the field trials.
Two experimental vineyards were used, one in Rovereto, Italy, and the other in Frick, Switzerland.¬† The Italian vineyard featured Cabernet Sauvignon vines on Kober 5BB rootstock (pergola trentina trellis system) and the Swiss vineyard featured Riesling-Sylvaner and Chasselas vines on Kober 5BB rootstock (guyot trellis system).¬† All soil and weed management practices were performed in ways approved for organic viticulture.
Each vineyard contained four replicates per treatment arranged in a randomized complete block design (great set-up, for you non-statisticians out there).¬† Each replicates‚Äô surface area was 18m2 and included 6 to 8 plants.¬† Each trial had an untreated control, and a standard treatment of copper hydroxide (in addition to the other compound treatments of interest).¬† In Italy treatments were sprayed at 7 day intervals, and in Switzerland, treatments were sprayed at 5-10 day intervals (dependent upon weather, plant growth, and downy mildew risk).
Disease incidence and disease severity were measured as they were in the indoor trials.¬† Leaves were assessed for disease every week in Italy, and 5 to 7 times per season in Switzerland.¬† To measure disease, 50 leaves and 50 bunches were randomly selected and analyzed.
- ¬† ¬† ¬†¬†There were 103 different experimental treatments, with a total of 218 various combinations and dosages.
- ¬† ¬† ¬† 24% (n=38) of these treatments were found worthy of field tests.
- ¬† ¬† ¬† Disease severity in the untreated control ranged from 17.8 to 71.6%.
- ¬† ¬† ¬† Disease severity in the copper hydroxide treatment ranged from 1.4 to 12.5%.
- ¬† ¬† ¬† Disease incidence in the untreated control ranged from 64.9-100%.
- ¬† ¬† ¬† Disease incidence in the copper hydroxide treatment ranged from 23.3-96.6%.
- ¬† ¬† ¬† 63.4% of the different treatments significantly inhibited disease development compared to the control.
- ¬† ¬† ¬† 36.5% of the different treatments were as effective as the copper hydroxide treatment.
- ¬† ¬† ¬†¬†Animal Origins:
o¬†¬† Chito plant, Enzicur, propolis, whey and grana significantly reduced downy mildew symptoms compared to the control.
- ¬† ¬† ¬†¬†Biocontrol Agents:
o¬†¬† Serenade and Trichodex were as effective as the copper hydroxide treatment.
- ¬† ¬† ¬†¬†Inorganic Materials:
o¬†¬† Armicarb, SaluKarb, Gro-stim, Kendal, and Ulmasud were as effective as the copper hydroxide treatment.
- ¬† ¬† ¬†¬†Microbial extracts and Natural Derivatives:
o¬†¬† Agat 25 K and Diamant were as effective treatments, except when applied at concentrations below 5-10%.
o¬†¬† GBA plant wash soap, Siva 50, and Tecnobiol significantly reduced downy mildew expression.
o¬†¬† Penergetic-p liquid and Phyto-Vital were the only natural derivative treatments that showed the same effectiveness as the copper hydroxide treatment.
- ¬† ¬† ¬†¬†Plant Extracts:
o¬†¬† Addit, BioZell 2000B, BM-608, Elot-Vis, Inulex, Jobeck Special Herb, licorice extract, Norporin BS liquid, Novosil, OA-21-0-N, Oak-ES, Plex-W, Quillaja liquid extract, Quiponin BS, R1, S1, S2, Saponin and Timorex all showed reduce downy mildew expression compared with the untreated control.
o¬†¬† Disease levels for all of these substances were equivalent to those in the copper hydroxide treatments.
- ¬† ¬† ¬†¬†Synthetic Materials:
o¬†¬† Beta-amino-butyric-acid, benzothiadiazole, and high levels of Tween 80 were as effective as the copper hydroxide treatments.
- ¬† ¬† ¬†¬†Other Copper Formulations:
o¬†¬† New copper formulas showed effectiveness similar to copper hydroxide, however, not in lower concentrations.¬†
¬ß¬† The level of downy mildew control decreased logarithmically to copper levels.
¬ß¬† The minimum level of copper necessary for effectiveness against downy mildew was 0.26g/L.
¬ß¬† Higher rates of copper did not increase the efficiency of the treatment.
- ¬† ¬† ¬† The treatment selection criteria for determining which treatment to use in the field trials were: 1) efficacy at low doses; 2) absence of phytotoxic effects in the indoor trials; 3) the price/economic efficiency of the material; and 4) the availability of the material in large enough quantities to use in vineyard management.¬† This resulted in the usage of 32 treatments from the indoor trials.
- ¬† ¬† ¬† Natural downy mildew infections were observed at both vineyards during all years of the study.
- ¬† ¬† ¬† In Switzerland, downy mildew killed 60-95% of the leaf canopy in untreated plants.
- ¬† ¬† ¬† In Italy, downy mildew killed around 60% of the leaf canopy in untreated plants.
- ¬† ¬† ¬†¬†Chito Plant, Serenade, Trichodex, Armicarb, Myco-Sin, Quiponin BS, S1, S2, salix extract, Saponin, Timorex, Agat 25 k, Zonix, Tecnobiol, Elot-vis, Inulex, and Enzicur all showed downy mildew control on bunches at greater than 60% control.
- ¬† ¬† ¬† Myco-sin was the best performer of the bunch, providing 70-99% disease control.
- ¬† ¬† ¬† The copper hydroxide treatment significantly protected the plants with control rates between 68 and 100% on leaves, and 61-99% on bunches.
- ¬† ¬† ¬†¬†Inulex, Chito Plant, and Naturam 5 showed toxic effects, and BM-608 + Trapper promoted disease development.
Basically, what the current study found was that without taking anything else into account, there are many compounds available that provide protection against downy mildew that is just as effective as the copper hydroxide sprays.¬† However, this study also found that many of these compound would not actually be useful as alternatives for copper in organic viticulture, as a result of other disadvantages that outweigh the advantages of disease control.
For example, some compounds are simply not produced in quantities large enough to be able to apply to a vineyard (let alone multiple vineyards) while other are simply much too expensive.¬† Other compounds (such as Myco-sin) showed phytotoxic effects down the grapes or other parts of the environment, which would result in a no-better alternative to an already environmentally-damaging copper spray.¬† Also, many compounds studied would have to undergo lengthy registration and bureaucratic hoops that would prove costly in the long run.¬† Many other compounds, while seemingly effective in the indoor trials, proved to be ineffective in the field trials, due to their poor ‚Äúrainfastness‚ÄĚ (in other words, they are ineffective in wet weather).¬† The paper described in detail many of the problems with a large number of the compounds reported in the results above, which I‚Äôll skip for now in lieu of space.
After all is said and done, this study found that the most promising candidates for replacing copper as an organic agent for protecting against downy mildew are the biocontrol agent, T. harzianum (Trichodex), and the plant extract, Y. schidigera (Norponin BS liquid), and S. officinalis (S2). ¬†These compounds displayed effectiveness comparable to the copper hydroxide treatments, and they did not display any negative side-effects that many other compounds displayed (i.e. phytotoxicity or poor performance in rainy weather). ¬†The authors contend that these alternatives warrant further study.¬† They also suggest that in the meantime, since they found that different formulations of copper were also effective, that organic viticulturists attempt to use these different formulas that contain lower levels of copper, until a suitable alternative can be found.
Please feel free to leave your comments/questions below, and I‚Äôll try to answer them to the best of my ability!
Source: Dagostin, S., Sch√§rer, H-J., Pertot, I., and Tamm, L. 2011. Are there alternatives to copper for controlling grapevine downy mildew in organic viticulture? Crop Protection 30: 776-788.
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