May i, 1889.] THE TROPICAL 
MILDEW AND ITS TREATMENT* 
Notice by Monsieur J. Dufour of Lausanne, 1888. 
The question of mildew has tor some years been of the 
highest importance for our vineyards. This new disease 
has spread with such rapidity, it has caused in many 
vineyards such great harm, that it is worth while to 
occupy oneself seriously about it, and to consider 
henceforth the application of sulphates as the fore most 
of the regular labours of the vineyard. 
We wish to try and summarize in the following 
pages the opinions gathered so far on the disease and 
its treatment, and that in a manner as concise as 
possible without enlarging much on the scientific 
characteristics of mildew or of the theories given out 
on the subject of the application of sulphates. Our 
aim is essentially practical. 
1 — Nature of tee Disease : Its Origin.— Mildew 
is a new disease introduced from America in the 
wake of so many others. It has ouly been 
known in Europe-since 1878. At the beginning of this 
year, it made rapid progress in France, and ended by 
invading the greater part of the vine-growing coun- 
tries of the Continent. 
In Switzerland, it must have made its appeai-anee 
already in 1880, but it was only some years later that at- 
tention was seriously drawn to this new enemy of our 
vineyards and to the means for combating it. 
0 rl A R A CTEKIST ICS OF THE DISEASE. — Yellow Spots, 
which appear first on isolated points of the leaf, consti- 
tute the first indica ions of the invasion of mildew. At 
the end of a short time, the spots become brown, and 
the tissue dries in the centre of the points of attack. 
When the disease declares itself severely, the isolated 
points uuite, and soon the leaf is found attacked on 
the greater part of its surface. Then it dries and 
finally falls. 
If one turns over the leaves attacked by mildew, one 
finds generally on the lower surface, on the parts which 
correspond with the yellow spots on the top of the leaf, 
a whitish powder, resembling finely-pulverised sugar. 
The Parasite. — The characteristic spots which have 
just been described are due to the development of a 
parasitic fungus, the Peronospora victicola. This fungus 
penetrates into the leaf, and lodges itself even in the 
thickness of the tissue. It nourishes itself at the 
expense of the juices of the leaf, and soon it throws 
into the air white filaments which produce seeds (or 
spores) „of ovoid form. These fructifications which 
appear in abundance, constitute precisely the whitish 
dust which is found on the lower face of the leaves 
attacked by miblew. The spores are easily carried by 
the wind, and germinate on other leaves of the vine: 
the fungus soon penetrates into the healthy leaf, and 
the attack propagates itself thus from vine to vine. 
At the end of autumn, the fungus may form other 
large seeds, the winter spores. It is often in this form 
that they live through the winter season, to reappear 
anew in the following year. 
Conditions of the Development of the Parasite. 
— It has been calculated that a single vine attacked by 
mildew can produce on the surface of its leaves several 
millions of spores, of which each one is capable of 
infecting new vines. With a power of reproduction 
so enormous, it is not surprising to see the disease 
spread with extreme rapidity. It may even appear 
surprising that it is not more, pernicious than it is in 
reality. But the development of mildew is happily 
limited by somewhat special conditions which the 
parasite requires in order to run through the different 
stages of its existence. It requires for its development 
heat, it requires also moisture. In order that the 
spores of mildew may germinate on the surface of the 
leaf, it is necessary that the latter should have been 
moistened either by rain or dew. 
For the fungus to grow and propagate itself rapidly 
in the interior of the leaves, sufficient heat is required. 
When these two conditions ■ are not found united, the 
disease does not develop in a manner sufficiently 
serious to compromise the growth. 
Times when the Disea.se may Appear. — If the con- 
ditions of which we have just spoken, heat and mois- 
* Referred to in letter of "J, G. C. H" on page 
745.-ED. L.B. 
96 
AGRICULTURIST. 763 
ture meet in the month of May, the disease will 
already appear at this time. This conjunction has not 
yet occurred with us, but it lias in Algeria and in the 
south of Fiance. In our country the mildew generally 
shows itself later. Lastyear, for examp e, ne trot 
leaves were found attacked towards the 10th July. 
To foretell in advance the precise time of the 
invasion of the disease, is to expose oneself to disagree- 
able surprises. Ail depends on meteorological circum- 
stances, impossible to foresee. 
Effects of Mildew. — Premature fall of the leaves; 
arrest of growth, incomplete maturity of the grapes, and 
vine-branches, such are the usual consequences of the 
disease. The vines are weakened in a sensible decree 
and resist frost less well in consequence. 
A vine attacked with mildew is recognised from 
far off from its yellow and partly dried up foliage. 
In general it is the leaves at the bottom of the stock 
which are found first attacked : the vines throw off 
little by little those leaves which are precisely the 
most necessary to nourish and protect the grapes. 
There exists besides a foim of mildew which is 
manifested on the grapes themselves. The parasite de- 
velopes in the clusters and denotes the appearance 
of the spots or rounded depressions, of a grey or 
violet colour, under which the pulp generally hardens. 
The alteration shows itself often in the first place 
in the neighbourhood of the peduncle (flower-stalk). 
The spore ends by shrivelling: it dries and falls. 
This form of mildtw has been often found in the 
Canton of Vaud. In America and France it is known 
under the names of "grey rot" or ' ; brown rot," 
and has caused great loss iu certain vine 1 ards. 
Possible Confusion with Other Diseases. — When 
the vine commences to bud and develop to a certain 
extent, one fim;s very trequently on tue leaves rounded 
spots, whitish below, often red or brown on tne 
upper side of the organ. Many persons mistake these 
spots for the commencement of an attack of mildew, 
but these alterations are due to a quite different 
cause— to the prick (fly-blow) of a little insect of 
the order acaritns, the Phytoptus vitis. 
This disease known by the name of erinose is 
easily distinguished by the following characteristics : 
the leaves are swollen and crimped in the parts 
attacked, but they do not usua'ly become yellow, and 
do not dry up rapidly as in the case iu mildew. In 
the leaves attacked by mildew, the white powder 
comes off with the least friction. In erinose the 
whitish spots are strongly adherent to the tissues 
of the leaf. They recommend against erinose repeated 
applications of sulphur. 
Another disease which may be confounded with 
mildew is brfflon. Brown spots, round or of irregular 
forms, appear on the leaves and the tissue dries up 
rapidly in the parts attacked. These spots are in 
general distinctly defined : they are not yellow in their 
circumference, as are those caused by mildew. On 
the lower face of the leaf, one sees no appearance 
of white efflorescence. The br&lon is due to the action 
of cold dews, and to the sudden changes of tem- 
perature. It is not a parasitic disease like mildew 
and erinose. 
The Treatment. Aim of Treatment : Action of 
Salts of Copper. — Once installed in the interior of 
the leaf, the parasite is invulnerable. In order to destroy 
it, one must destroy equally the leaf which encloses it. 
Preventive Treatment is then the only possible 
one. The spores of mildew must be prevented from 
germinating on the surface of the leaves : the parasite 
must be prevented from penetrating the plant. After 
numerous fruitless attempts, we have succeeded at 
last within the last few years, in discovering modes 
of treatment which are positively efficacious. It was 
in 1884 that attention was first drawn on several 
sides to the valuable property which salts of copper- 
possess in acting energetically against the development 
of the parasite and in killing the spores, or at least 
in preventing their normal development. 
The aim of the treatment is to spread on the 
leaves salts of copper in a soluble form before the 
arrival of the mildew. There remain thus on the 
leaves spots constituting true reservoirs of copper, 
