296 
JOURNAL OF HORTICULTURE AND COTTAGE GARDENER. 
September 27, 1894. 
POTATO-TOP COLLAPSE. 
The disease that has destroyed the leaves and haulm of many Potatoes 
throughout the country this year is primarily caused by the conidial 
condition (Macrosporium commune, Rahh.') of the mature fungus called 
Pleospora herbarum, and is common on the decayed portions of various 
plants. But Macrosporium commune is not the only aggressor, for 
other species of Macrosporium are present on the tops of Potatoes— 
namely, M. sarcinula, Berh.^ and M. tomato. M. sarcinula attacks 
outdoor (also indoor when the houses are cold and damp) Cucumbers, 
the disease being characterised by retardation of growth and curling of 
the leaves, especially in the young vines, and the stunted fruits, 
with yellowness in the whole plants and final collapse, when they 
become brown or black, and completely withered. M. sarcinula is also 
common on withered grass leaves, from whence the disease no doubt 
passes to Cucumbers and even Vegetable Marrows. Neglected hedges 
and ditches are in the main responsible, combined with cold and moist 
weather, for the prevalence and spread of the disease. 
M. tomato, Coohe, forms blackish patches on Tomato fruits, especi¬ 
ally those nearly ripe, but it also attacks the flowers or rather the 
ovaries, and prevents the setting of the fruits. It also has a penchant 
for Potato “ apples,” and when it does attack the Potato crop it is at 
or soon after the commencement of flowering. It not only prevents the 
formation of flowers but spreads to the leaves and haulm. The leaves 
ti.^'sues by the mycelium, each bearing “ conidia variable in form, oblong, 
obovate or clavate, attenuated at the base, 3—5 septate, septa transverse, 
oblique or longitudinal, olivaceous, episcore sometimes granular ” 
{Massee), These conidia germinate readily, and, in suitable conditions 
—moisture and a host affording the proper nourishment—produce 
mycelium, which reproduces the fungus. 
Prevention only is possible, and may be effected by spraying with 
Bordeaux mixture just before the Potato plants come into blossom, or 
when showing, and again in about ten days. Half strength Bordeaux 
mixture is quite strong enough to destroy the germ tubes of the Macro¬ 
sporium spores, but as the Potato fungus (Phytophthora infestans) 
generally follows the blossoming of the Potatoes it would be wise to use 
the full strength Bordeaux mixture at the third spraying, or three weeks 
after the first at half strength for the Macrosporium, and thus both 
Macrosporium commune, which destroys the tops only, and Phytophthora 
infestans attacks may be entirely prevented. Cleanly culture, the 
avoidance cf rank farmyard or stable manure, and abundance of room for 
the plants, together with silicates for strengthening the haulm, are 
valuable aids in avoiding both diseases. 
Remedies there can be none, as the mycelium of the fungus is 
internal in its operations. All diseased parts, however, should be 
burned, as these contain the Pleospora herbarum more or less, though 
not to a great extent at present (September 10th), but it will come more 
abundantly presently, and by thus proceeding the disease producing 
power of the parasite (it has hitherto been sneered at as a 
saprophyte) will be proportionately reduced. 
Magnum Bonum has wholly collapsed in some cases, but 
some have yet green tops more or less, on which we found 
Phytophthora infestans, so that diseased tubers may yet have 
to be added to the disasters. Satisfaction and Abundance 
have held their ground quite as well or better than Magnum 
Bonum, while “ The Bruce ” has fairly healthy-looking foliage. 
The fungus (Macrosporium) however, has “ got hold ” of it— 
robust and strong constitutioned as it is—and it is only s 
question of a few days for the parasite to compass its destruc¬ 
tion, especially if the weather prove cold and wet, and it is 
being aided by Phytophthora infestans, which we found on 
specimen examined on September 8th.—G, Abbey. 
As bearing on this important subject, and apparently in 
confirmation of what has been written by our correspondent, 
we reproduce the following notes and woodcuts from a small 
pamphlet entitled “ Some Destructive Potato Diseases: What 
They Are, and How to Prevent Them,” published by the 
United States Department of Agriculture. We have also seen 
large breadths of Potatoes both in Kent and Surrey with the leaves 
blackened and destroyed, yet the tubers are sound though small. 
“ Among the many fungous diseases affecting the Irish 
Potato three are especially worthy of consideration on account 
of the regularity of their occurrence in the United States and 
the widespread losses they occasion. The diseases in question 
• are Potato blight or downy mildew, the Macrosporium disease, 
and Potato scab. In the accompanying pages are given some 
of the more important characters, by means of which the 
several diseases may be distinguished, together with brief direc¬ 
tions embodying the latest information in regard to treatment. 
How THE Diseases may be Distinguished. 
LEAP OP POTATO AFPECTED WITH BLIGHT (PHYTOPHTHORA INFESTANS). 
commence curling, assume a sickly hue, and brown or black patches 
appear on them and the stems. Soon after this the tops wither, 
hanging flaccid and blackened, as if smitten by frost. This form 
may occasionally be found on the common Nightshade (Solanum 
dulcamara) so prevalent in hedgerows. 
M. commune, however, is most frequently found on Potatoes, and 
its attacks usually commence when the haulm has arrived at the 
flowering development, but a plant here and there may take the disease 
and collapse at an earlier period of the growth, some such sickly plants 
occasionally being found at earthing up time. If the weather is wet 
and cold the disease may spread rapidly from these over a whole field, 
parish, county, or country. The plants look luxuriant for a brief time, 
owing to the action of the fungus, ultimately turning a sickly hue, black 
spots or rather patches appear on the leaves and run together, then the 
leaflets cither drop or the whole leaf droops, turns black and shrivels. 
The tops follow suit partly or wholly down, the stem having some 
blackish spots here and there, which are the seat or have been of the 
Macrosporium, and it is followed by the Pleospora herbarum. This 
fungus being the final stage is only found on decaying or dead stems 
and leaves, whence the conclusion is arrived at that the fungus only 
attacks plants that are unhealthy. This is an excellent thing for the 
spread and perpetuation of the fungus, for which the grower pays, 
reaping only half or three parts of a crop of half or three-quarters 
grown tubers, but they are sound unless Phytophthora infestans intervene 
as it is now doing in some cases. Macrosporium commune (other Macro- 
sporiums are similar) first appears as a bluish or greenish-grey coat of 
erect filaments on the sickly plants, made so by permeation of the 
Potato Blight ^Phytophthora infestans) —This disease 
(fig. 43) attacks the leaves, stems, and tubers. Generally the 
first noticeable effect upon the leaves is the sudden appearance 
of brownish or blackish areas, which soon become soft and 
foul smelline. So sudden is the appearance of the disease in some 
cases that fields which one day look green and healthy may within 
the next day or two become blackened as though swept by fire. The 
rapid spread of the disease is dependent in large measure upon 
certain conditions of moisture and heat. A daily mean or normal 
temperature of from 72° to 74° F. for any considerable time, accom¬ 
panied by moist weather, furnish the best conditions for the spread of 
the disease. On the other hand, if the daily mean or normal tempera¬ 
ture exceeds 77° for a few days the development of the disease is 
checked. This fact exp’ains why the Potato blight fungus seldom 
ojcurs to any serious extent in sections where the mean or normal 
daily temperature exceeds for any length of time 77°. The tubers 
affected with the disease in question show depressed, dark coloured 
spots on the surface, while within are seen blotches and streaks of a 
brownish or blackish colour. Other diseases may produce similar effects, 
so that in this case the changes are not so characteristic as those shown 
by the leaves. 
" The Macrosporium Potato Disease. —From reliable evidence there 
is every reason to believe that this disfase is often more widespread and 
destructive in this country than the true blight. There is no doubt that 
the two diseases are often mistaken for the same thing ; in fact, it has 
been the custom for many years to attribute nearly all maladies affecting 
the Potato to the blieht fungus. 
“ The Macrosporium disease (fig. 44) attacks the leaves and sometimes 
the stems, but never the tubers. The disease may appear at any time 
after the plants? are from 4 to 6 inches high. At first the older leaves 
show greyish brown spots, the affected parts becoming hard and brittle. 
