134 JOUKNAL OF THE EOYAL HOETICULTUEAL SOCIETY. 
surface of the leaf. On this surface is found the myceHum of the fungus, 
consisting of a great number of white, dehcate, creeping, septate hyphae, 
much branched and interwoven. On this mycehum innumerable upright 
branches, or conidiophores, are produced, at the apex of which unicellular 
spores, or conidia, are produced in a long chain in basipetal succession. 
(See fig. 35.) Myriads of these conidia are produced, and soon cover 
the under-surface of the leaves with a white powdery meal. The conidia 
are capable of immediate germination, and being carried by the wind or 
insects to fresh beds quickly spread the disease. In this conidial, or 
Oidium stage, the present fungus can be easily distinguished from the 
Rose Mildew {S. 2^CLn7iosa), as in the latter the conidia are longly elliptic 
or sub -cylindrical, and measure about 30x 14 ^ (see fig. 35), while in the 
Strawberry Mildew the conidia are much wider and of a different shape, 
and measure 30-35 x 20-24 ^. (See fig. 35.) 
The mycelium sends suckers, or haustoria, into the epidermal cells of 
the leaf, and by means of these extracts its nourishment from the Straw- 
berry plant. Eventually reddish-coloured patches appear on the under- 
surface of the leaves, which remain after the mycelium has disappeared : 
these are due to epidermal cells killed or affected by the fungus. 
Soon after its appearance on the leaves the mildew, in bad attacks, 
begins quickly to spread to the fruit. The berries in all stages of develop- 
ment are attacked : the green fruit when overrun by the fungus begins 
to dry up, and does not colour nor reach maturity, while the ripe fruit 
continues juicy, and, except in the severest attacks, when the whole berry 
is white as though dusted with flour, remains more or less red. The 
first indication of the fungus on the fruit is usually to be found in the 
presence of mycelium and conidiophores on the " seeds " (achenes), from 
which centres the fungus gradually spreads and invests the whole of the 
succulent part of the fruit. The fungus is truly parasitic on the fruit, 
sending haustoria into the epidermal cells of the succulent parts between 
the achenes. (See fig. 36.) 
Later in the season the fungus produces a second form of fruit. This 
consists of perithecia, each of which contains a single ascus with eight 
ascospores. I have not yet seen this form of fruit on English Straw- 
berries, but Prof. J, C. Arthur has kindly sent me examples from America 
(now in the Kew Herbarium). The characters shown by these specimens 
refer the fungus to SphcBrotheca Humuli (DC.) Burr. The perithecia 
occur on the petioles of the leaves, and are only very sparingly produced. 
A group of three of these perithecia are represented at fig. 36, together 
with an ascus and ascospores. The cells of the perithecial wall (fig. 36) 
average about 15 ^ wide. 
Varieties Attacked. — It is stated in the Journal of the Board of 
Agriculture (G) that " all kinds of Strawberries were equally affected." 
In the outbreaks of the disease observed this year, however, it certainly 
appeared as though some varieties were, as regards their fruit, more or 
less immune from the disease. Thus in one case, while the fungus 
quickly ran over the fruits of ' Paxton ' and ' British Queen,' the fruit of 
both the ' Noble ' and the ' Royal Sovereign ' escaped. This was all the 
more remarkable because the plants of the two last-named varieties were 
grown in close proximity to those of the other varieties ; yet, although in 
