UNIVERSITY OF CALIFORNIA AGRICULTURAL EXPERIMENT STATION BENJ. IDE WHEELER, PRESIDENT COLLEGE OF AGRICULTURE thomas forsyth hunt, dean ano director H BERKELEY VAN NORMAN, VlCE-Di rector and Dean University Farm School CIRCULAR No. 144 (December, 1915) OIDIUM OR POWDERY MILDEW OF THE VINE By FREDERIC T. BIOLETTI and F. C. II. FLOSSFEDER The Oidium, usually called Mildew on the Pacific Slope, is the only serious fungous disease of the vine in California. It has been found in every vineyard section. In some, the cooler and moister districts, it does great damage every year to all vines not properly treated; in others, the hotter and drier districts, it does little damage except to very susceptible varieties and in years when the weather is favorable to its development. Fig. 1. — White patches of Oidium on upper surface of young leaf. The generally recognized standard treatment for the Oidium is to dust the vines with sulfur. Many growers find this treatment perfectly efficacious and satisfactory. Many, on the other hand, complain that it is not sufficient or even that it is useless. In consequence, many other methods of treatment are being suggested and tried. The observations of the station tend to show that the causes of failure are two. First, the treatment is not properly applied. The sulfur used is sometimes of poor quality and, very often, the time and method of application are ill chosen and defective. Second, other diseases and troubles of the vine for which sulfur is useless are often mistaken for mildew and the treatment is condemned when it fails to cure or prevent these diseases. The other methods of treatment suggested are of two classes : ( 1 ) Winter spraying of the dormant vines and (2) Liquid sprays in the growing season with various fungicides. All these methods have been tested by many investigators with the practically uniform result that they have been condemned as useless or as inferior to dry sulfuring. An account of some of the evidence leading to this conclusion is given in Bulletin 186 of this experiment station — "The Oidium of the Vine." Dissatisfaction with sulfuring Fig. 2. — Blackening of canes due to Oidium. and attempts to discover something better continue, however, and some comparative tests were made last year at Davis with some of the remedies most talked about. The results may be stated briefly as follows: 1. Winter spraying is useless. 2. Liquid sprays are more costly and less efficient than dry sulfuring. 3. Dry sulfuring properly applied is efficient. COMPARATIVE TESTS OF SULFURING, LIQUID SPRAYS AND WINTER TREATMENT Sulfuring. — A good commercial brand of sulfur was used and ap- plied with efficient knapsack bellows. The work was done as nearly as possible as it would be done in practice by an energetic and intelli- gent man. Owing to the small size of the treated plots, the work was done a little more rapidly than it would be on a large vineyard. A test cm a 15 acre vineyard indicates that the labor' cost would be about 50 per cent higher than is shown in the experiments, but that, the amount of sulfur used would be the same. This indicates that the total cost w r ould be about 25 per cent greater, as the labor and sulfur costs were about equal. Liquid sprays. — Many liquid sprays have been used in the attempt to control oidium. The most successful have been those whose result is to leave fine particles of sulfur on the leaves. Some of the type of the lime sulfur mixture, contain polysulfides which break up after they reach the leaves and leave a residue of finely divided sulfur. Others are simply "wetable sulfurs" or finely ground sulfur mixed with small quantities of inert matters which cause the sulfur to mix easily with water and to spread and adhere to the leaves. A spray of the latter type was used and applied with a good knapsack sprayer. Winter Treatment. — The idea of winter treatment is to destroy any spores or other resting forms of the fungus which may remain on the dormant vine after the fall of the leaves. One of the most effective sprays for this purpose and that most frequently recommended and used by grape growers is a simple water solution of blue stone (copper sulfate). A 2 per cent solution was used and applied very thoroughly with a Bean pow r er spraying machine just before the swell- ing of the buds in spring. Test Plot. — The experiments were conducted on a block of 1078 vines planted 6 X 12 feet (49 rows of 22 vines each) in a well isolated vineyard of about 3% acres. The arrangement of the tests and the comparative injury from mildew are shown in the following list : Mildew None Slight Bad Very bad Bow ] o 3 4 5 6 8 9 10 \12 13 11 15 Variety Burger Burger Burger Burger Burger Burger Burger Burger Burger Burger Carignane Carignane Carignane Carignane Carignane Treatment Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Wet Sulfur Wet Sulfur Wet Sulfur Wet Sulfur Wet Sulfur Wet Sulfur Winter Spraying Winter Spraying Winter Spray Winter Spray Winter Spray Winter Spray Winter Spray Bad Slight Very slight Slight Slight None Mildew Bow Variety Treatment 16 Carignane Dry Sulfur 17 Carignane Dry Sulfur 18 Carignane Dry Sulfur 19 Carignane Dry Sulfur 20 Carignane No treatment 21 Cornichon No treatment Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur Dry Sulfur '36 Tokay , Wet Sulfur 37 Tokay Wet Sulfur 38 Tokay • Wet Sulfur 39 Tokay Wet Sulfur 10 Tokay Wet Sulfur Wet Sulfur Wet Sulfur Wet Sulfur Wet Sulfur Wet Sulfur '46 Muscat Dry Sulfur 47 Muscat Dry Sulfur 48 Muscat Dry Sulfur 49 Muscat Dry Sulfur 22 Cornichon 23 Cornichon 24 Cornichon 25 Cornichon 26 Cornichon 27 Cornichon 28 Cornichon 29 Cornichon 30 Cornichon '31 Tokay 32 Tokay 33 Tokay 34 Tokay 35 Tokay 41 Muscat 42 Muscat 43 Muscat 44 Muscat 45 Muscat The plot includes five varieties of various degrees of susceptibility from the extremely susceptible Carignane to the moderately suscept- ible Burger. The presence of two untreated rows (20 and 21) of Carnignane and Cornichon near the middle of the plot insured an abundant supply of spores to infect the whole plot. The dry and wet sulfur were so arranged that the effect of each could be observed separately and in conjunction with winter treat- ment. The effect of winter treatment alone was also tried. The following summary gives the kind and date of treatment and observations on the amount of mildew present on July 20, 1915. On rows marked "badly mildewed" the crop was practically a total loss, on those marked "slightly mildewed" the loss was not serious. Exp. 1. Burger, rows 1-4 — Dry Sulfur May 7, May 20, June 16, July 7, No Mildew. Exp. 2. Burger, rows 5-6 — Winter Spray March 15, Dry Sulfur, May 7, May 20, June 16, July 7, No Mildew. Exp. 3. Burger, rows 7-10— Wet Sulfur May 7, May 20, June 16, July 7, Slightly Mildewed. Exp. 4. Carignane, rows 11-12 — Winter Spray March 15, Wet Sulfur May 7, 20, June 16, July 7, Badly Mildewed. Exp. 5. Carignane, rows 13-15 — Winter Spray March 12, Very Badly Mildewed. Exp. 6. Carignane, rows 16-19 — Dry Sulfur May 7, 20, June 15, July 7, Badly Mildewed. Exp. 7. Carignane, row 20 — No treatment, Badly Mildewed. Exp. 8. Comiehon, row 21 — No treatment, Badly Mildewed. Exp. 9. Comiehon, rows 22-30 — Dry Sulfur May 7, 20, June 16, July 7, Slightly Mildewed. Exp. 10. Tokay, rows 31-35— Dry Sulfur May 7, 20, June 16, July 7, Very Slightly Mildewed. Exp. 11. Tokay, rows 36-40— Wet Sulfur May 7, 20, June 16, July 7, (More Mildew than in Exp. 10, B. 31-35). Exp. 12. Muscat, rows, 41-45 — Wet Sulfur May 7, 20, June 16, July 7, Slightly Mildewed. Exp. 13. Muscat, rows 46-49— Dry Sulfur May 7, 20, June 16, July 7, No Mildew found except a very little on one bunch of one vine. COMPARISON OF COST OF SULFURING AND SPRAYING VINES Sulfuring, per Acre — S. lbs.d) Minutes(2) Total cost May 7 1.72 $0,043 22.5 $0,075 $0,118 May 20 1.72 0.043 23.4 0.078 0.121 June 16 4.40 0.110 26.5 0.088 0.198 July 7 6.87 0.172 29.0 0.093 0.265 Totals 14.71 $0,368 101.4 $0,334 $0,702 Spraying, per Acre- Spray gals.(:: ) Minutes (2 j Total cost May 7 7.01 $0,063 33.5 $0,112 $0,175 May 20 9.70 0.087 35.3 0.118 0.205 June 16 9.70 0.087 41.0 0.137 0.224 July 7 13.80 0.124 50.0 0.167 0.291 Totals 40.21 $0,361 159.8 $0,534 $0,895 (1) Sulfur reckoned at $2.50 per 100 pounds. (2) Time reckoned at 20 cents per hour. (3) Wet sulfur reckoned at $7.50 per 100 pounds as purchased or 9 <•< per gallon for the spray used. The vines were in their third year, well grown and very uniform in size. They were all short pruned and of medium height. The amount of sulfur used and the time taken in applying it are unusually small and may be taken as the minimum for four sulfurings of a bearing vineyard. With trellised or very high vines even with competent workmen the cost would be nearly double. In another experiment vineyard of 13 y 2 acres at Davis five sul- furings were given at a cost of $1,064 per acre. This vineyard con- tained several hundred varieties of vines of various ages and sizes. About 75 per cent were bearing and from three to five years old, the remainder were one or two years old. Many styles of pruning were represented, including two-wire trellises and high cordons. In spite of these unfavorable conditions and the presence within a few hundred feet of untreated and affected vines there was practically no crop loss from mildew. It is not uncommon for grape growers to use twenty, thirty, or more pounds of sulfur per acre at a cost for material and labor of over $1.00 for each sulfuring. Most of this sulfur is wasted and often the mildew is not controlled, owing to defective methods and inappropriate times of application. SUMMARY OF RESULTS OF TESTS 1. Four dry sulfurings, i.e., two before, one during and one after blossoming, controlled the mildew perfectly on varieties of average susceptibility, (Burger and Muscat), even when the conditions were exceptionally favorable to the development of the fungus. The chief of these conditions are a source of abundant spores for reinfestation and suitable weather for their germination and growth. (See Experi- ments 1, 2 and 13.) The proximity of two untreated rows (rows 20 and 21) of Carig- nane and Cornichon insured an abundant and continuous supply of spores. The weather of early summer was comparatively moist and cool, a condition favorable to the fungus in the interior valleys. The mean temperature at Davis for the month of May was 60.7 or 7.2 degrees below the normal and 71.2 for June or 3.6 degrees below normal. 2. Under the same conditions, with more susceptible varieties (Cornichon and Tokay), the same treament controlled the mildew fairly well. (See Experiments 9 and 10.) 3. With a variety of maximum susceptibility (Carignane) none of the treatments has any perceptible effect. (See Experiments 4, 5, 6, 7 and 8.) 4. The wet spray (wet sulfur 12 pounds in 100 gallons of water) used was somewhat less effective than the dry sulfur. (Compare Experiment 1 with 3; 10 with 11; 13 with 12.) 5. Spraying with copper sulfate solution just before the starting of the buds (winter spraying) was unnecessary. (Compare Experi- ment 1 with 2.) 6. Spraying in winter had no effect. (Compare Experiments 4 and 5 with 6 and 7.) 7. The cost of perfect control with four dry sulfurings was 70 cents per acre; that of partial control with wet spraying 90 cents or 28.6 per cent more. 8. The cost of each treatment increased with the lateness of the season. Calling the cost of the first dry-sulfur treatment 100 per cent, the cost of later treatments was as follows: Dry sulfur at blossoming 167.8 per cent, after setting 224.4 per cent; wet spray at blossoming 190 per cent, after setting 246 per cent. CONCLUSIONS 1. Mildew on most varieties can be controlled cheaply by proper sulfuring, even under unfavorable conditions in the interior valley. 2. It is probable that it can be controlled on all varieties by the same means in the same region. 3. The presence of missed or untreated vines much increases the difficulty of controlling the disease on all the vines. 4. Winter treatments are unnecessary and useless. 8 5. Wet sprays are more expensive to apply, and that tested less effective than dry sulfur. It is probable that other wet sprays would be no more or even less effective. DIRECTIONS FOR SULFURING VINES Number of Sulfurings- sulfur will depend on the -The number of times it is necessary to locality, the variety, the season and the Fig. 3. — Vine at right stage of growth for the first sulfuring. sources of infection and will vary from one to six. In the interior valleys two will usually be enough if the work is done regularly and properly in all the vineyards. The first sulfuring should be done when the shoots are not more than six or eight inches long. (See Fig. 3.) Later, the foliage becomes dense and it is difficult to cover every part of the surface with sulfur. At the first sulfuring it should be possible to see daylight through all parts of the vine. The first sulfurings are preventive and are the most important and effective. If we wait until mildew is perceived, the cost of control will be much greater and it will often be impossible to control the disease completely. The second sulfuring should be given during or just before blossom- ing. At this time the interior of the mass of foliage will be dense in places, but the first sulfuring has already covered the parts which might escape the second. With Muscats and most wine grapes these sulfurings will usually give perfect control if there are no summer irrigations and no untreated vines or parts of viues in the immediate vicinity. With very susceptible varieties, such as the Carignane, or excessively vigorous growers, like the Emperor, or where summer irrigation is practised, a third sulfuring may be necessary about the time the grapes are half grown. Only imperfect work or abundant reinfection from untreated vines should necessitate more than three treatments except in the coast regions of summer fogs and even here three will usually be sufficient. Time of Sulfuring. — The time of day is of no importance, provid- ing there is not so much wind as to prevent proper distribution or so much moisture that the leaves tend to stick together and escape the sulfuring. Dry, moderately warm weather is the best, but the first sulfuring should be done even if such weather does not occur. Distribution of the Sulfur. — It should be the aim in sulfuring to eradicate absolutely every vestige of mildew from the vineyard. This can be done only by placing sulfur on every exposed surface. If a single leaf in the vineyard escapes it may produce enough spores to reinfect the whole. This complete distribution is easily accomplished at the first sulfuring if done at the time recommended. It is possible but more difficult, during blossoming and practically impossible later. It is for this reason that the early sulfurings are the most important. Proper distribution can be accomplished only with very fine sulfur, which easily floats in the air, and with a distributing machine which throws it out in a spreading cloud. The object should be to produce a cloud of light sulfur dust which will float through the vine and leave the particles of sulfur distributed on every minute piece of surface. This cannot be done with can shakers or sulfur bags, except by the use of- unnecessarily large amounts of sulfur. Some form of blower which will thoroughly break up the adhering masses of sulfur particles and expel them with sufficient force is needed. Suitable machines are obtainable in California, but all in use are of foreign manufacture. 10 Kind of Sulfur. — The finer the sulfur the more perfect the distri- bution. Most of the sulfurs which have been offered to the grape- growers during late years are of good quality, judging by the samples received at the station. Some have a considerable amount of coarse material which may be considered waste. Lately some exceedingly fine-ground sulfurs have been produced which are undoubtedly excel- lent. Some sulfurs, while fine, have a tendency to form lumps which are difficult to break up and are thrown out entire by some machines. Such lumps are completely wasted and increase the amount of sulfur needed considerably. Examination of Sulfur. — The grower can easily determine the suitability of a sulfur by a few simple tests. 1. A very good idea of the fineness of a sulfur can be obtained by means of an ordinary good hand magnifying glass, costing about $1.00. A half-inch Coddington lens is suitable. "With a little experi- ence sublimed can be distinguished from ground sulfur by this means. The particles of sublimed sulfur are spherical and with smooth outlines ; those of ground sulfur are irregular and with angular outlines. The sulfur is most easily examined when spread between two pieces of glass (microscopical slides) and held over a dark background. A minute pinch of the sulfur is placed on one piece of glass and spread out by rubbing it with the other. It spreads more perfectly if first moistened with a little ether or gasoline. 2. With a fine sulfur there should be little or no grit felt when it is rubbed on the palm of the hand. 3. If the sulfur is suborned the deeper yellow the color the finer it is. Ground sulfur, on the contrary, is lighter or whiter the finer it is. 4. The finer the sulfur the more bulky. With sacks of equal weight the larger will contain the better sulfur. 11 Fig. 4. — Sublimed Sulfur of very good quality Fig. 5. — Sublimed Sulfur of poor quality. 12 1*^1 ■&&.T4*'** ?#V> Fig. 6. — Ground Sulfur of good quality. Fig. 7. — Ground Sulfur of very bad quality