Spraying Plums for Brown Rot.

PROF. E. C. STAKMAN, MINN. EXP. STATION, UNIVERSITY FARM, ST. PAUL.

The brown rot of plum is without doubt one of the important limiting factors in plum-growing in Minnesota. In seasons favorable to its development, losses of from twenty to fifty per cent. of the crop in individual orchards are not uncommon.

Experiments on the control of the disease have been carried on by the sections of "Plant Pathology and Tree Insects and Spraying," of the Minnesota Experiment Station, since 1911. No accurate results could be obtained in 1912 and 1915 on account of crop failure in the orchards selected for experiment. Results are available for the years 1911, 1913 and 1914.

Brown rot is caused by a fungus (_Sclerotinia cinerea (Bon.) Wor._).

Every plum grower knows the signs of the disease on the fruit. Blossoms, leaves and twigs may also be affected. The diseased blossoms become brown and dry, and fall from the tree; the diseased leaves become brown and may die. Young twigs may also be killed.

Infection may occur at blossoming-time. The amount of blossom blight depends very largely on weather conditions; in fairly warm, moist weather there is usually more than in drier weather. The same is true of the rot on the fruit; during periods of muggy weather it may spread with amazing rapidity. The rot does not usually attack the fruit until it is nearly or quite ripe, although green plums may rot, especially if they have been injured. It is important to know that a large percentage of rotted plums have been injured by curculio. Counts have shown that in many cases as much as eighty-five per cent. of the rot followed such injury.

Rotted plums should be destroyed for two reasons: (1) The spores produced on them may live during the winter and cause infection in the spring; (2) if the mummies fall to the ground, late in April or early in May of the second spring the cup fungus stage may develop on them. This cup fungus produces a crop of spores capable of causing infection.

Spraying experiments, the summarized results of which are given here, show that the disease can be fairly well controlled even in badly affected orchards.

Some of the experiments were carried on in the orchards at University Farm and some in commercial orchards. There were from twelve to forty-five trees in each plot, and the trees on which counts were to be made were selected before the rot appeared. The percentages given below refer to fruit rot and do not include blossom or twig blight. The object was to determine the times for spraying and the most effective spray mixtures. Details are for the most part omitted, and the results of various experiments are averaged.

For convenience the times of spraying are designated as follows:

1. When buds are still dormant.

2. When blossom buds begin to show pink.

3. When fruit is size of a pea.

4. Two weeks after third spraying.

5. When fruit begins to color.

It did not pay to apply Spray 1. In the plots on which applications 1, 2, 3 and 4 were made there was an average of 6.3 per cent. of rot, while in those from which Spray 1 was omitted there was an average of 6.7 per cent. rot, a difference so slight as to be negligible. Neither did Spray 4 seem to pay, there being an average of 10.9 per cent. brown rot when it was applied and 11.4 per cent. when it was omitted.

The schedule finally adopted was therefore the application of Sprays 2, 3, and 5. Spray 2 is necessary to prevent blossom blight, although it has not always reduced the amount of rot on the fruit. Spray 5 is the most important in reducing the amount of rot. In all of the experiments during three years the average amount of rot in the sprayed plots which did not receive Spray 5, was 10.7 per cent. On the plots which received Spray 5, with or without the other sprays, the average amount of rot was 4.6 per cent., and the average on unsprayed plots was 34.8 per cent.

Excellent results were sometimes obtained by applying only Spray 5, although this did not, of course, have any effect on blossom blight. In 1913 the amount of brown rot in one plot which received only Spray 5 was 3.3 per cent., while in the unsprayed plots it was 33.9 per cent. In 1914 the amount of rot was reduced from 38.8 per cent. in unsprayed plots to 6.5 per cent. in the plots to which Spray 5 was applied.

Possibly Spray 3 could be omitted without seriously interfering with results; success in controlling the rot with Spray 5 alone seems to indicate this. It was hoped to settle the matter during the past summer, but spring frosts spoiled the experiment.

For the present it seems advisable to recommend the application of Sprays 2, 3, and 5. In the first two, two and a half pounds of a.r.s.enate of lead paste, or one and one-fourth pounds of the powder should be added to each fifty gallons of spray mixture in order to kill the curculio. In the plots sprayed in this way in 1911 ninety-six per cent.

of the fruit was perfect, while in the unsprayed plots only 81.6 per cent. was perfect, and in 1913 and 1914 the amount of brown rot was reduced from 34.8 per cent. to 4.6 per cent. Several growers have reported excellent results from these three applications, and there is no reason why other growers should not duplicate them.

[Ill.u.s.tration: Brown rot of plums showing the small, grayish brown tufts of spores. Can be controlled by destroying mummies and thorough spraying.]

The efficiency of various fungicides was tried. Self-boiled lime-sulphur, 8-8-50; commercial lime-sulphur, 1 to 40; 2-4-50 and 3-4-50 Bordeaux; iron sulphide made up with 1 to 40 commercial lime-sulphur, and iron sulphide made up with 10-10-50 self-boiled lime-sulphur were tried and all gave good results. Commercial lime-sulphur, 1 to 40, has been used in commercial orchards with excellent results, and it will probably be used more than the other spray mixtures because it is so easy to use. Possibly weaker solutions of lime-sulphur would do just as well as 1 to 40. This will be determined, if possible, during the summer of 1916.

Good results were obtained only when a high pressure was maintained in spraying. There was a clearly observable difference between plots sprayed with low pressure and those sprayed with a pressure of more than 175 pounds. For large orchards a power sprayer is desirable; for small orchards a barrel sprayer with an air-pressure tank attached is large enough. Such an outfit can be bought for $35 or $40 and can do good work.

The cost of spraying three times should not exceed fifteen cents a tree.

The results from spraying orchards which contain a great deal of brown rot and have never before been sprayed will probably not be so good the first year as in better kept orchards, but by spraying regularly each season the disease can be well controlled.

Mr. Cashman: Please state what you mean by 3-4-50 there.

Mr. Stakman: 3-4-50 Bordeaux mixture means three pounds of bluestone or copper sulphate, four pounds of lime, and fifty gallons of water. The copper sulphate should be dissolved in twenty-five gallons of water, the best way being to put it into a sack and hang the sack in the water. The lime should be slaked and then enough water added to make twenty-five gallons of milk of lime. Here is where the important part of making up the spray comes in. Two people should work together and pour the milk of lime and the bluestone solution together so that the streams mix in pouring. It is very important that the mixing be thorough and the mixture should be used fresh.

The President: Do you add any Paris green at any time or a.r.s.enate of lead?

Mr. Stakman: Always add a.r.s.enate of lead two times, when the buds are swelling and when the plums are the size of green peas.

The President: How much?

Mr. Stakman: I would rather leave that to Professor Ruggles. We used from 2-1/2 to 3 pounds and Mr. Ruggles, I think, found 2-1/2 pounds was enough.

The President: That is, 2-1/2 pounds to 50 gallons of water with the other ingredients?

Mr. Stakman: Yes.

Mr. Dyer: I would like to ask if you have ever used a.r.s.enate of lead for spraying plums?

Mr. Stakman: In the experiments which we conducted in co-operation with Mr. Ruggles, of the Division of Entomology, we always used a.r.s.enate of lead in the first two sprayings to kill the curculio.

Mr. Dyer: I had quite an experience, so I want to know what your experience was.

Mr. Stakman: We never had any trouble with it.

Mr. Dyer: I have had an experience of thirty years, and I have never seen or had on my place any brown rot, and I never was troubled with any curculio, and I practically always used a.r.s.enate of lead.

Mr. Cashman: Isn't it a fact if you begin spraying your plum trees when they are young and spray them early, at the right time, you have very little trouble with the brown rot? And spray them every year?

Mr. Stakman: Yes, that is it. You might be disappointed the first year if the orchard had never been sprayed, but by spraying year after year you finally cut it down.

Mr. Cashman: You said a pressure of 200 pounds ought to be used?

Mr. Stakman: Yes, but it isn't necessary to get an expensive power sprayer to keep up that pressure. There are sprayers on the market that cost from $30 to $40 which have a pressure tank by which the pressure can be maintained at from 175 to 250 pounds without any great amount of trouble, that is, for a small orchard. If you have a big enough orchard for a power sprayer, of course get it.

Mr. M'Clelland: This summer my plum trees, the leaves all turned brown and came off. What is the reason?

Mr. Stakman: When did it happen?

Mr. M'Clelland: Along in August, I think; July or August.

Mr. Stakman: What kind of soil were they on?

Mr. M'Clelland: Clay.

Mr. Stakman: Did you spray?

Mr. M'Clelland: Yes, sir, I sprayed.

Mr. Stakman: What did you use?

Mr. M'Clelland: Lime-sulphur, I think.