Irregular shaped black blotches on the older leaves of the plant is likely to be Black Spot, followed by yellowing of the remainder of the leaves, which drop prematurely. When a bad infestation the whole plant can lose all its leaves which leads to a weakening of the plant, a number of the branches may die back.
A fungal problem caused by warm sunny days with cool dewy nights and high humidity. Mostly occurs in spring and autumn. Black Spot thrives in high humidity and warmth so a major problem on the tropical areas of Australia especially the east coast and northern semi-tropical areas of Australia. Some varieties have little resistance, such as yellow and orange coloured roses. Others, like some Rugosa’s are immune to it.
Best controlled with a systemic fungicide spray ‘Mancozeb Plus’ or ‘Eco-rose’ sprayed onto the leaves which can be used every 10 days if necessary. This can be found in many products such as ‘Rose Guns’, ‘Blackspot Control’, and ‘Rose Fungal Spray’. ‘Eco-Oil’ or ‘Pest Oil’ is also recommended for control. Spray in the cool of the morning. If overhead watering the plants, do early in morning so foliage will dry out during the day, so reduce humidity. Dripper-line watering is preferred.
Black spots show on the topside of the leaves, which turn yellow and drop off. If not treated the whole plant may be defoliated. When temperatures rise in mid-summer the problem is reduced and can disappear with hot weather. More likely to be seen on yellow and orange coloured varieties yet can occur on all but the healthiest varieties. Blackspot is one of the only problems which can ruin the fun of growing roses. When planning a new garden ask about blackspot resistance roses from us. If a variety is highly susceptible to this problem maybe it should not be purchased.