Mention was made in the Gardeners' Monthly for March (p. 74) of a disease that has proved very destructive during the past winter to the leaves and young shoots of greenhouse roses. Some specimens sent me by Mr. Meehan show that the trouble is due to a fungus related to that which causes the potato rot. Most of the leaves sent me were soft and flabby, and very tender, as if beginning to rot. At first no other unusual appearance was evident, excepting a brownish, unhealthy color; but a hand lens shows that their lower surface is covered in places with a fine gray mold, which is nowhere very plentiful, but forms scattering little tufts just visible to the naked eye. Under a compound microscope these tufts are found to consist of slender threads .3-.5 min, (1/75 - 1/125 inch) high, which are straight and un-branched to near the top, where they fork regularly five or six times, the long, curved branches ending in round or slightly oval spores, measuring .013X-.0165 to .016X.02 min. (.0005-.0007x.0007-.0008 inch). The threads, which are .006-.013 min. (.0002-.0005 inch) in diameter, come from an irregular mycelium in the leaf, and emerge through the stomata singly, or in clusters of two or three, rarely more.

They are grayish, while their spores, which are very numerous, are decidedly gray, and form the most noticeable part of the fungus. On very badly diseased leaves they appear abundantly on both surfaces, coming up through nearly all the stomata. The figure gives an accurate representation of both the mycelium and fruiting plant, magnified 350 diameters. This fungus, which is now reported for the first time as causing a serious disease in this country, was noticed a few years ago on wild roses in California. It was first studied in England, where it was destructive to winter roses in 1862, by Dr. Berkeley, the eminent president of the Royal Horticultural Society, who described it in the Gardeners Chronicle for that year (p. 308), under the name Peronospora sparsa. Wittmack also found it very injurious in the rose houses of Lichtenberg, Germany, in 1876 and 1877*. The disease is spread in two ways: by bits of mycelium in cuttings taken from affected plants, and by the conidia or spores, which grow directly into new mycelia. No resting spores, like those of the lettuce and grape molds, have ever been found. Sulphur promises little relief, for it is already used on roses by most gardeners to keep down the true rose mildew.

If the red spider will allow it, growing the plants in a dryer house may be beneficial; but the only treatment that is at all likely to be of value is to thoroughly destroy every branch that bears diseased leaves, cutting it back some distance below where it appears perfectly sound, and carefully dropping it into a pail of water containing some disinfectant, such as a little corrosive sublimate, from which it may be removed to the fire. If the disease reappears at once, the entire plant is affected, and ought to be destroyed. Care should be taken to prevent the introduction of diseased plants of anything from the greenhouses where the fungus is known to exist. Madison, Wis., May 9, 1884.

Peronospora sparsa.

Peronospora sparsa.

(Berkeley) from leaf of cultivated rose (X350).

Scattering little tufts just visible to the naked eye. Under a compound microscope these tufts are found to consist of slender threads .3-.5 min, (1/75 - 1/125 inch) high, which are straight and un-branched to near the top, where they fork regularly five or six times, the long, curved branches ending in round or slightly oval spores, measuring .013X-.0165 to .016X.02 min. (.0005-.0007x.0007-.0008 inch). The threads, which are .006-.013 min. (.0002-.0005 inch) in diameter, come from an irregular mycelium in the leaf, and emerge through the stomata singly, or in clusters of two or three, rarely more. They are grayish, while their spores, which are very numerous, are decidedly gray, and form the most noticeable part of the fungus. On very badly diseased leaves they appear abundantly on both surfaces, coming up through nearly all the stomata. The figure gives an accurate representation of both the mycelium and fruiting plant, magnified 350 diameters. This fungus, which is now reported for the first time as causing a serious disease in this country, was noticed a few years ago on wild roses in California. It was first studied in England, where it was destructive to winter roses in 1862, by Dr. Berkeley, the eminent president of the Royal Horticultural Society, who described it in the Gardeners Chronicle for that year (p. 308), under the name Peronospora sparsa.

Wittmack also found it very injurious in the rose houses of Lichtenberg, Germany, in 1876 and 1877*. The disease is spread in two ways: by bits of mycelium in cuttings taken from affected plants, and by the conidia or spores, which grow directly into new mycelia. No resting spores, like those of the lettuce and grape molds, have ever been found. Sulphur promises little relief, for it is already used on roses by most gardeners to keep down the true rose mildew. If the red spider will allow it, growing the plants in a dryer house may be beneficial; but the only treatment that is at all likely to be of value is to thoroughly destroy every branch that bears diseased leaves, cutting it back some distance below where it appears perfectly sound, and carefully dropping it into a pail of water containing some disinfectant, such as a little corrosive sublimate, from which it may be removed to the fire. If the disease reappears at once, the entire plant is affected, and ought to be destroyed. Care should be taken to prevent the introduction of diseased plants of anything from the greenhouses where the fungus is known to exist.

Madison, Wis., May 9, 1884.

As a remedy for the rose trouble referred to as Philadelphia Rose Rot, but which was also troublesome in places throughout the West and elsewhere, I would suggest plenty of fresh pure air. In my experience this is the doctrine I have always kept in view, believing plants require fresh pure air as necessary to health, the same as people, and have never seen any such destructive fungus in our houses, although we had upwards of a thousand roses planted on the benches of one of the houses last winter, and all our plants have always been praised for their health and vigor; which we relate not boastfully, although with some pleasure, but for the purpose of bearing us out in the doctrine stated.

We have observed florists as a general rule keep their houses too close during winter, especially a severe one such as the last was. They do nor avail themselves of the bright and warm days to midday, to admit fresh, pure air into the greenhouses. The result is a stagnant and impure atmosphere, kept so by confinement, the sunlight having no power to purify a confined and motionless atmosphere, which at once invites and encourages fungus growth and other low forms of vegetable life - the parasitic enemies of the higher types. The more highly organized forms of vegetable life all require light, and pure fresh air, as vitally essential to vigor, while on the other hand semi-darkness, and a moist stagnant atmosphere seem requisite to the vegetation, luxuriance and reproduction of the lower forms, of which the troublesome rose fungus, Peronospora sparsa, described and illustrated by Prof. Trelease, in his valuable article on pages 211 and 212 of the July number, is a representation. So, what we would suggest as obviously the radical enemy or remedy and preventative, is in plenty of pure, fresh air and sunshine.

New Albany, Ind., July 8, 1884.