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.