Purple and Orange Rust of Blackberry
Purple Rust (Phragmidium violaceum) and Orange Rust (Phragmidium bulbosum)
Both types of rust infect cultivated blackberries and are also found on wild blackberries in Croatia. They are widespread in all temperate zones where blackberries are grown. Purple rust is more common and has surprised growers in some years, as blackberries were not protected until the appearance of yellow stem rust (Kuehneola uredinis).
Affected Part: Leaves
Pathogens: Fungi Phragmidium violaceum and Phragmidium bulbosum.
Symptoms: On the upper side of the leaf, yellow-red clusters (spermagonia) appear. About ten days later, aecia about 1 mm in diameter form. In the case of P. violaceum, the aecia are reddish or purplish with a yellowish center, while for P. bulbosum, they are yellow to orange. Shortly thereafter, on the underside of the leaf, darker clusters (uredosori) appear. Uredosori of P. bulbosum are yellow and 0.1 to 0.3 mm in diameter, while those of P. violaceum are orange-yellow and 0.2 to 1 mm in diameter. Depending on the infection, chlorosis of part of the leaf, curling, and leaf drop may occur. Towards the end of the growing season, the clusters turn brown. The shoots may also become infected, forming small cankers.
Biology of the Disease: Both parasites are transmitted from one growing season to another via teliospores, which form abundantly on the leaves in the fall. Teliospores germinate to produce basidia with four basidiospores. Basidiospores infect young blackberry leaves. At the infection site, the first spermagonia form, soon followed by aecia on the leaf underside. Spermagonia and aecia form in May on the leaf surface or shoots and can also appear on the fruit stalks. Aeciospores are released from the aecia, infecting the leaves. About ten days after infection, the first uredosori form, typically in June. Uredosori produce uredospores, which cause secondary infections and are dispersed by the wind over long distances. The first teliosori appear in August. In addition to this usual method, the fungus can also overwinter as a mycelial stroma on infected shoots.
Conditions for Infection: Severe infections by uredospores occur at temperatures of 18 to 29 °C with continuous leaf wetting for 18 hours or more. The number of secondary infections depends on the climatic conditions that influence the severity of the infection.
Protection Against Purple and Orange Rust of Blackberry: Protection should be implemented on susceptible varieties after the appearance of the first leaves, and for less susceptible varieties after the first spermagonia appear. The fungicides used to successfully control these rusts are the same as those used for controlling yellow rust of blackberry shoots.
Yellow Rust of Blackberry Shoots and Leaves
Kuehneola uredinis
This rust has become a problem in Croatia since the introduction of thornless blackberries. It is present in all areas where thornless blackberries are grown. Early infections can cause premature leaf loss, resulting in insufficient ripening of the shoots and their freezing during the winter.
Affected Part: Stems, leaves, rarely fruit.
Pathogen: Fungus Kuehneola uredinis.
Symptoms: The most noticeable symptoms appear on the shoots. During the winter and early spring, swellings can be seen on the bark, slightly darker purple in color. In these spots, the bark begins to crack in April and May, forming elliptical cracks 3 to 5 mm long. Uredosori form in the cracks, releasing uredospores like golden-yellow powder. A single shoot can have multiple cracks with uredosori, sometimes merging and extending up to several centimeters. On the underside of leaves, usually in June or later, small round yellow clusters appear. Severe leaf infections, which are not common in Croatia, cause yellowing and leaf drop. Towards the end of the growing season, teliosori form between the uredosori on the underside of the leaves, appearing as lighter yellow. Aecia mostly form on the upper side of the leaf or on the shoots, located along the veins and in large ring-shaped clusters surrounding the spermagonia in a concentric pattern.
Biology of the Disease: This rust has a complete life cycle on the same host. Teliospores are viable immediately after formation, producing a basidium on which basidiospores form. Basidiospores can infect shoots and leaves in the fall. At the infection sites, spermagonia with spermatia form, followed by aecia with aeciospores. From a practical standpoint, the important overwintering form is the mycelium. It persists in the shoots from one season to the next, producing uredospores in the spring. Uredospores spread the infection during the growing season.
Protection Against Yellow Rust of Blackberry Shoots and Leaves: Avoid planting highly susceptible varieties such as Thornfree and Evergreen. It is also important to prune and burn infected shoots after harvesting. Preventive spraying against a complex of diseases should begin before the growing season starts and can continue until flowering using copper-based fungicides. Specific protection should begin in the spring as soon as the first symptoms are observed. Cyproconazole, which also controls leaf rusts, is effective.
Purple Blotch of Blackberry Shoots
Septocyta ruborum
This disease has been reported in our region and causes significant damage in older plantations.
Affected Part: Shoots
Pathogen: Fungus Septocyta ruborum.
Symptoms: During the summer, dark green spots about 2 cm in diameter appear on the basal part of the shoots, and symptoms also appear on the lateral shoots and the upper part of the shoots. The spots become reddish, then brown, elongated, and irregular in shape. The spots are bordered by a thin, more intense reddish zone that separates them from the green part of the shoot bark. The spots merge, covering several internodes. Infected shoots become stunted, and heavily infected shoots dry up along with the leaves, flowers, and possibly fruits. Black structures (pycnidia) can be seen within the spots.
Biology of the Disease: The parasite survives as mycelium in the shoots. By spring, pycnidia form, maturing in April. Pycnospores are released from April to August. Pycnospores germinate at temperatures from 0 to 33 °C, with an optimum of 21 to 24 °C. Pycnospores can only infect current-year shoots. The incubation period is long, with symptoms appearing in summer.
Protection Against Purple Blotch of Blackberry Shoots: New plantations should be established using healthy planting material. If infections are observed in the plantation, infected shoots should be cut and burned. Before the growing season starts, copper-based fungicides should be applied.
Chestnut Blotch of Blackberry Shoots
Didymella applanata
This fungus parasitizes Rubus species. In our region, the most significant damage occurs on raspberries and blackberries. Notable damage was observed after the establishment of larger plantations.
Affected Part: Leaves, shoots
Pathogen: Fungus Didymella applanata.
Symptoms: Changes in the plants are observed at the end of June and the beginning of July. Symptoms appear on the leaves of first-year shoots, starting from the leaf margin and spreading towards the main vein, forming a triangular-shaped lesion with a dark brown color and a yellowish border. Older leaves, shaded leaves, parts of the plant closer to the ground, and parts exposed to dew are more frequently affected. As the fungus spreads in the leaf, premature leaf drying occurs. The infection spreads from the petiole to the shoot, where spots form around the dormant buds in the leaf axils. These spots are slightly darker green than the surrounding healthy tissue. Later, they become purplish and, in the final stage, chestnut-brown, sometimes zoned, covering a larger or smaller part of the internode. They are elliptical in shape. If a spot girdles the shoot, it dries out. In the fall, when the canes start to lignify, the symptoms become less noticeable. During winter, part of the bark dies and takes on a silvery color. On these areas, black spheres embedded in the bark tissue can be seen. The bark cracks and peels off the shoots. Buds on infected shoots survive, but in the next spring, they often do not sprout, and if they do, they remain stunted.
Biology of the Disease: The fungus overwinters as mycelium in the infected shoots under the bark and as pseudothecia and pycnidia. Pycnidia are present almost throughout the year, and pycnospores are released from March to November, causing primary infections on the leaves. During spring and summer, a considerable number of pseudothecia with ascospores form. Ascospores develop in pseudothecia in April, but the highest number of ascospores is released during May and June. Ascospores are released during rainy weather, and pycnospores are released during high relative humidity and rain. Infections are more severe if the shoots are exposed to moisture for longer periods.
Protection Against Grey Mold in Blackberries
There are differences in susceptibility among varieties. Protection should be applied just before flowering, with a second spraying immediately after flowering. Effective fungicides for controlling grey mold in strawberries can also be used for blackberries.
