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Even though one may cure a very bad infestation of mildew, the plant suffers considerably from the effects of the fungus. Such attacks result only from planting roses too much in the shade, or choosing varieties that are very susceptible to mildew, plus a failure to realize, well in advance, the likelihood of this occurring, and neglecting to use an appropriate preventive spray. Plants affected to this extent produce very poor blooms, if any; those that are only slightly affected may give roses of almost usual quality.

Downy mildew affects roses, but is distinct from the form described, and does not exist in Australia or New Zealand. Mildew of other plants is due to slightly different fungi and cannot be transmitted to roses.

black spot (Plate 42)

Black spot, a fungus disease due to Diplocarpon rosae, is apparent only on mature leaves and stems, though it may gain entry while the growth is young. Black or brown spots appear on the foliage, then the remainder of each leaflet turns yellow, and it soon drops. Frequently, whole plants of susceptible varieties become defoliated; many then die back badly, and in some instances the whole plants die. Black spot infestations reach their maximum in mid- or late summer, and are much worse in some years than in others. The disease is most common in humid coastal areas, and least common in our inland districts, where the atmosphere is dry. It seems probable that soil conditions are important too, but we do not know yet whether this is so, and, if so, in what way. Therein may lie the explanation of why roses raised in Barcelona and on the French Riviera do not get much black spot there, whereas they are so badly affected here that they will not grow in our coastal areas, where the climate is similar. Some rose-growers state that black spot is worst in acid soils, and support their claims by pointing out that the soil round Barcelona is alkaline. They overlook several other considerations, especially the very efficient natural drainage of that area. Other people consider that roses do better in every way in slightly acid soils.

Tea roses are almost immune to black spot; Pernetianas are least resistant. Very susceptible varieties should be avoided.

The spores of black spot shelter during the winter in bark crevices and in the soil. The fungus penetrates into the substance of the leaves and stems, rendering it quite inaccessible. Hence cure is impossible and prevention is the only hope of control. Maintenance of adequate soil-moisture, especially by mulching, helps considerably in reducing the incidence of black spot; and so does avoidance of overhead watering after midday, for if rose foliage remains wet for six hours or more the spreading of all fungus diseases is helped.

The first leaves seen to be affected with black spot should be clipped off and burnt. At any time of the year all rose stems and leaves should be burnt as soon as possible after cutting, for they carry at least a few spores of black spot and mildew.