Cane and Leaf Diseases

Anthracnose

Anthracnose is caused by the fungus Elsinoe veneta. One of the most common and widespread diseases of brambles in the United States, anthracnose can infect red and black raspberries, blackberries, dewberries, and loganberries. The disease is very destructive on black and purple raspberries. On red raspberries, it can be common but is usually not a serious problem. Disease losses can occur from defoliation, general stunting and a decrease in cane vigor, reduction in fruit yield and quality, and cane death. Resistance to anthracnose is not available in most varieties. The use of fungicide (lime sulfur) and cultural practices such as sanitation (removal of old and infected canes) are key control methods.

Symptoms

Anthracnose can cause symptoms on canes, leaves, fruit and stems of berry clusters. The most striking symptoms are on canes. A few days after the fungus invades the succulent tissue of young canes, minute purplish spots appear. These spots enlarge in diameter and become oval or lens-shaped. The centers become somewhat sunken and are pale-buff to an ash-gray color (Figure 27). Margins are somewhat raised and purple to purple-brown. If numerous, the lesions may merge and cover large portions of the cane. Diseased tissue extends down into the bark, partly girdling the cane. As the canes dry in late summer and early fall, diseased tissue often cracks. In the following year, fruit produced on severely diseased canes may fail to develop to normal size and may shrivel and dry, especially in a dry growing season.

Figure 27: Anthracnose lesions on black raspberry canes.

On leaves, anthracnose appears on the upper surface in early- to mid summer as irregular, yellowish-white spots about 1/16 inch in diameter (Figure 28). The spots gradually enlarge and develop a reddish-purple margin around a light-gray center. The centers of these spots may drop out, producing a "Shot hole" effect. This "Shot hole" symptom is more common on trailing blackberries and raspberries. On blackberries, leaf spots may merge together producing large grayish dead areas between the veins. Anthracnose does not usually cause much damage to leaves of erect blackberries.

Figure 28: Anthracnose leaf symptoms on black raspberry.

Disease Development

The anthracnose fungus overwinters in the bark or within lesions on infected canes (Figure 29). In early spring the fungus produces two types of microscopic spores called conidia and ascospores. Conidia, which are produced in small fungal fruiting structures called acervuli, are the most common form of inoculum. Ascospores are comparatively rare. Production of these spores coincides with the leafing out of brambles in early spring. Spores are rain-splashed, blown, or carried by insects to young, succulent, rapidly growing plant parts that are susceptible to infection. The spores germinate in a film of water and penetrate into the plant tissue. Symptoms appear about a week later. Small pimple-like reproductive bodies are produced within lesions on infected canes and the fungus overwinters there. These bodies produce conidia for new infections the next spring, completing the disease cycle. As canes age and harden, they become much less susceptible

Figure 29: Anthracnose disease cycle. Disease cycle of Raspberry Anthracnose. Taken from the Compendium of Raspberry and Blackberry Disease and Insects of the American Phytopatholical Society. Used with permission.

Cane Blight

Cane blight is caused by the fungus Leptosphaeria coniothyrium. Cane blight is one of the more damaging diseases of raspberries. The disease is most common on black raspberries, but also occurs on red and purple cultivars. Cane blight occasionally occurs on blackberries and dewberries. Cane blight can result in wilt and death of lateral shoots, a general weakening of the cane, and reduced yield. It is usually most severe during wet seasons. The fungus often invades the cane through wounds. Any practice that reduces wounding on canes is beneficial for control. Key control methods are the same as for anthracnose.
 

Symptoms

Dark brown to purplish cankers form on new canes near the end of the season where pruning, insect, and other wounds are present. The cankers enlarge and extend down the cane or encircle it, causing lateral shoots to wilt and eventually die (Figure 30). On second-year canes, the side branches may suddenly wilt and die, usually between blossoming and fruit ripening. On close examination, dark brown or purplish cankers can be observed on the main cane or branches below the wilted area. Infected canes commonly become cracked and brittle and break easily. Tiny black specks (pycnidia), which are reproductive bodies of the cane blight fungus, develop in the brown cankered bark. In wet weather, large numbers of microscopic spores (conidia) ooze out of the pycnidia. This ooze gives the bark a dark-gray, smudgy appearance.

Figure 30: Cane blight lesion on thornless blackberry.

Disease Development

The pathogen survives over winter on infected or dead canes (Figure 31). The following spring, conidia, formed in the pycnidia, ooze from them during wet periods, and are blown, splashed by rain, and carried by insects to nearby canes. Under moist conditions, the spores germinate and penetrate into the plant through pruning wounds, insect punctures, fruit stem breaks, and other wounds. After entry, the fungus rapidly invades and kills bark and other cane tissues. Pycnidia are formed in older cankers and complete the disease cycle. Dead canes can continue to produce conidia and remain a source of infection for several years.

Figure 31: Cane blight disease cycle. Taken from the Compendium of Raspberry and Blackberry Diseaes and Insects of the American Phytopathological Society. Used with permission.

Spur Blight of Red Raspberries

Spur blight is caused by the fungus Didymella applanata. Spur blight occurs only on red and purple raspberries. Spur blight has been considered to be a serious disease of red raspberry; however, recent studies in Scotland suggest that spur blight actually does little damage to the cane. The extent of damage caused by spur blight in the United States is not clearly understood. Key control methods are the same as for anthracnose and cane blight.

Symptoms

Symptoms first appear on young canes in late spring or early summer. Purple to brown areas (cankers) appear just below the leaf or bud, usually on the lower portion of the stem (Figure 32). These cankers expand, sometimes covering all of the area between two leaves. In late summer or early fall, bark in the cankered cane area splits lengthwise and fungal fruiting bodies, appearing as small black specks, develop in the cankers. They are followed shortly by the formation of many slightly larger, black, erupting spots, another form of fungal fruiting body. Leaflets sometimes become infected and show brown, wedge-shaped diseased areas, with the widest portion of the wedge at the top of the leaf. Infected leaves may fall off, leaving only petioles without leaf blades attached to the cane (Figure 33).
 

Figure 32: Typical symptoms of spur blight on red raspberry canes.

Figure 33: Symptoms of spur blight on red raspberry leaves. The "V-shaped" lesions are characteristic.




As diseased primocanes become fruiting canes (floricanes) during the next season, the side branches growing from diseased buds are often weak and withered and produce less fruit.

Disease Development

The fungus survives the winter in diseased canes (Figure 34). The following spring and summer, during wet and rainy periods, spores are released and carried by splashing rain and wind to nearby new growth. There they germinate and produce new infections, where the fungus will again overwinter.

Figure 34: Spur blight disease cycle. Taken from the Compendium of raspberry and Blackberry Diseases and Insects of the American Phytopathological Society. Used with permission.

Septoria Leaf and Cane Spot

Septoria leaf and cane spot is caused by the fungus Septoria rubi. The disease is common and can be quite severe in the southern portions of the Midwest on erect and trailing blackberries and black raspberries. Leaves and canes of severely infected plants become badly spotted. The disease can cause premature defoliation which will produce weak plants that are more susceptible to winter injury.

Symptoms

On leaves, Septoria leaf spot lesions have a whitish to gray center surrounded by a brown to purple border (Figure 35). The spots are circular and are about 1/8 inch in diameter. Tiny black pycnidia (fungal fruiting bodies) form in the center of the spots. The pycnidia are small; therefore, it may be necessary to use a magnifying glass (10X hand lens) to see them. Leaf spots caused by Septoria are similar to those of anthracnose. Spots on canes and petioles are similar to those on leaves but are generally more elongated.

Figure 35: Septoria leaf spot on blackberry leaflet.

Disease Development

The fungus overwinters as mycelium and pycnidia (fungal fruiting bodies) in dead plant debris (leaves and stems) and on infected canes. Pycnidia on infected canes from a nursery can be an effective means for moving the fungus into new fields. In the spring, spores (conidia) are produced inside the pycnidia. They are released in high numbers and carried to young susceptible leaves and canes by splashing or wind-driven rain. The fungus spores germinate in a film of moisture and penetrate the leaf or cane tissue. As leaf and cane spots form and age, new pycnidia form in the centers. These also produce and release spores that can cause secondary infections throughout the growing season. Although the environmental conditions required for infection are not clearly understood, periods of rainfall are highly conducive to disease development. After overwintering in infected canes or debris, the fungus produces spores for new infections the following spring, completing the disease cycle.

Rosette

Rosette, or double blossom, is caused by the fungus Cercosporella rubi. Rosette is a serious disease of many varieties of erect and trailing blackberries, particularly in the humid southern United States and the southern regions of the Midwest. The disease is not present in the more northern regions of the Midwest. Rosette also occurs on red and black raspberry but is seldom serious. Rosette infected blossoms do not form berries and non-infected parts of the same cane may produce poor quality fruit. The disease seriously reduces fruit quality and yield. Once the disease is established in organic plantings, little can be done to control it.

Symptoms

Symptoms of rosette disease are striking and may completely change the plants' appearance due to a proliferation of shoots. This proliferation of shoots is referred to as a witches' broom. Buds on new canes of erect and trailing blackberries are infected in early summer. Generally, no symptoms will develop until the following spring, although a few "Witches' brooms" may develop during warm spells in late fall. In the spring, numerous leafy sprouts develop from infected buds (Figure 36). These shoots are generally smaller than normal and have pale green foliage that later turns a bronze color. Several of these witches' brooms may be formed on one cane. Unopened infected flower buds are abnormally large and coarse and frequently somewhat redder. Sepals enlarge and occasionally change into leaves. Flower petals may become green and leaflike. As flower buds open, petals are usually pinkish in color, wrinkled and twisted. Pistils are usually larger and longer than normal and occasionally become abnormally shaped. The fungus produces a whitish spore mass that can cover the surface of the infected pistils and stamens. Berries do not develop from infected blossoms. Non-infected parts of the same cane often produce small, poor quality fruit. In some varieties the witches' brooms symptoms may not be apparent; however, the fruit set in infected blossoms is always impaired.

Figure 36: Symptoms of rosette disease on blackberry.

Disease Development

Young buds on vegetative canes are infected in early spring (Figure 37). The double blossom fungus grows between the bud scales and surrounds the embryonic tissues within the bud. As secondary buds develop beside an infected bud, they are also invaded. After the bud is colonized by the fungus, very little happens. Infected buds usually remain symptomless until the next spring. A few infected buds are sometimes forced out in an unusually warm late fall. The fungus overwinters in infected buds. During the winter the fungus continues to grow within the bud. Bud proliferation is induced. When infected buds break dormancy in spring, they develop a large number of short, abnormal and off-colored shoots (the witches' broom effect). Infected flower buds usually produce abnormal blossoms upon which the fungus produces its spores. These spores are carried by wind or insects to the newly formed vegetative buds, which are only susceptible to infection in early spring. The fungus infects these buds and overwinters in them to cause new symptoms the next spring, thus completing the disease cycle.

Figure 37: Disease cycle of rosette. Taken from the Compendium of Raspberry and Blackberry Diseases and Insects of the American Phytopathological Society Used with Permission.

Powdery Mildew

Powdery mildew is caused by the fungus Sphaerotheca macularis. Powdery mildew affects susceptible cultivars of red, black, and purple raspberries. Blackberries and their hybrids are usually not affected. The disease can be severe (varying from year to year) on highly susceptible cultivars, and these plants may be stunted and less productive. The infection of flower buds reduces fruit quantity. Infected fruit may be lower in quality or unmarketable as a result of the unsightly covering of mycelial growth. The key control method is to avoid susceptible varieties. Sulfur will provide good control on susceptible varieties.

Symptoms

Infected leaves develop light green blotches on the upper surface. Generally, the lower surface of the leaf directly beneath these spots becomes covered by white, mycelial growth of the powdery mildew fungus. The leaf spots may appear water-soaked. Infected leaves are often mottled, and if surface growth of the fungus is sparse, they often appear to be infected by a mosaic virus. Infected shoot tips may also become covered with mycelial growth (Figure 38). When severely infected, the shoots become long and spindly (rat-tailed), with dwarfed leaves that are often curled upward at the margins (Figure 39). Infected fruit may also become covered with a white mycelial mat. When the disease is severe, the entire plant may be stunted.

Figure 38: Powdery mildew on blackberry leaves. Note the leaves are covered with the white growth of the fungus.

Figure 39: Powdery mildew on blackberry. Note the distortion of leaves.

Disease Development

The fungus overwinters as mycelium in buds on shoot tips in Minnesota, but in California it has been reported to overwinter only as cleistothecia (fungal fruiting structures), producing ascospores as primary inoculum in the spring. Conidia are generally abundantly produced on the surface of infected tissue, and these serve as secondary inoculum for repeated cycles of infection throughout the growing season. They are airborne and probably remain viable for no more than 21 days. The development of this disease, like most other powdery mildew diseases, is favored by warm, dry weather.

Orange Rust

Orange rust is the most important of several rust diseases that attack brambles. All varieties of black and purple raspberries and most varieties of erect blackberries and trailing blackberries are very susceptible. Orange rust does not affect red raspberries. Orange rust is caused by two fungi that are almost identical, except for a few differences in their life cycles. Arthuriomyces peckianus occurs primarily in the northeastern quarter of the United States and is the causal agent for the disease in the Midwest. Gymnoconia nitens is a microcyclic (lacks certain spores) stage of A. peckianus. G. nitens is the more common rust pathogen on erect and trailing blackberries in the Southeast.

Unlike all other fungi that infect brambles, the orange rust fungus grows "Systemically" throughout the roots, crown and shoots of an infected plant, and is perennial inside the below-ground plant parts. Once a plant is infected by orange rust, it is infected for life. Orange rust does not normally kill plants, but causes them to be so stunted and weakened that they produce little or no fruit. Key control methods are cultural practices such as removing infected plants early in the spring and eradication of wild hosts (brambles) near the planting. Organic fungicides are not effective for control. In severely infested areas, black raspberries or blackberries should probably not be planted. Red raspberries are not susceptible.

Symptoms

Orange rust-infected plants can be easily identified shortly after new growth appears in the spring. Newly formed shoots are weak and spindly (Figure 40). The new leaves on such canes are stunted or misshapen and pale green to yellowish (Figure 41). This is important to remember when one considers control, because infected plants can be easily identified and removed at this time. Within a few weeks, the lower surface of infected leaves are covered with blister-like pustules that are waxy at first but soon turn powdery and bright orange (Figure 42, 43). This bright orange, rusty appearance is what gives the disease its name. Rusted leaves wither and drop in late spring or early summer. Later in the season, the tips of infected young canes appear to have outgrown the fungus and may appear normal. At this point, infected plants are often difficult to identify. In reality, the plants are systemically infected, and in the following years, infected canes will be bushy and spindly, and will bear little or no fruit.

Figure 40: Black raspberry plants showing early season symptoms of orange rust. Note the "Spindly" elongated shoots. Orange pustules will develop on the underside of infected leaves.

Figure 41: Leaves on infected plants are usually yellow (chlorotic) and smaller than leaves on healthy plants.

Figure 42: Orange rust symptoms on the underside of a black raspberry leaf.

Figure 43: Close-up blister like pustules on the underside of an infected black raspberry leaf. Pustules contain bright orange masses of fungus spores.

Disease Development

In late May to early June, wind and perhaps rain-splash spreads the bright orange aeciospores from the pustules on infected leaves to healthy susceptible leaves where they infect only localized areas of individual mature leaves (Figure 44). When environmental conditions favorable for infection occur, the spores germinate and penetrate the leaf. About 21-40 days after infection, small, brownish black telia develop on the underside of infected leaflets. The teliospores borne in these telia germinate to produce a basidium, which in turn produces basidiospores. In blackberries these spores then infect buds on cane tips as they root. They also may infect buds or new shoots being formed at the crowns of healthy plants in the summer. The fungus becomes systemic in these young plants, growing into the crown at the base of the infected shoot, and into newly formed roots. As a result, a few canes from the crown will show rust the following year. The fungus overwinters as systemic, perennial mycelium within the host.

Figure 44: Orange rust disease cycle. Taken from the Compendium of Raspberry and Blackberry Diseases and Insects of the American Phytopathological Society. Used with Permission.

Orange rust is favored by low temperatures and high humidity. Temperatures ranging from 43 to 72 F favor penetration and development of the fungus, but higher temperatures decrease the percentage of spore germination. At 77 F, aeciospores germinate very slowly, and disease development is greatly retarded. Spore germination and plant penetration have not been observed at 86 F. Aeciospores require long periods of leaf wetness before they germinate, penetrate, and infect plants.

Late Leaf Rust

Late leaf rust, caused by the fungus Pucciniastrum americanum, can cause serious damage to susceptible red raspberry cultivars. Economic losses occur from fruit infection and premature defoliation. Because it usually appears late in the season and only occasionally in a severe form, some consider it to be a minor disease. The wild red raspberry, Rubus strigosus, in the eastern United States is very susceptible to this rust. A number of cultivars originating from this species also are highly susceptible. While late leaf rust occurs throughout the northern half of the United States and southern Canada, it is more common east of the Mississippi River. In recent years, its occurrence has increased in the northern areas of the Midwest and it has caused significant losses. The rust does not occur on black raspberries or blackberries. Organic fungicides are not effective for control. Once established in the planting, little can be done to control it.

Symptoms

On mature leaves, late leaf rust causes small chlorotic or yellow spots to form on the upper leaf surface (Figure 45). These spots may turn brown before leaves die in the fall. Unless the disease is severe, foliar infections can be rather inconspicuous. Small pustules filled with powdery spores (not waxy like orange rust spores) are formed on the undersides of infected leaves (Figure 46). These spore masses may also occur on leaf petioles, canes, and even on the fruit. Infected fruit are worthless; thus, yield of marketable fruit is reduced (Figure 47). Badly infected leaves may drop prematurely, and in years when the disease is severe, canes may be defoliated by September.

Figure 45: Symptoms of late leaf rust on the upper surface of red raspberry leaves. Note the chlorotic spots.

Figure 46: Symptoms of late leaf rust on the lower surface of red raspberry leaves. Note the masses of powdery yellow spores.

Figure 47: Late leaf rust symptoms on red raspberry fruit. Note the pustules on individual drupelets.

Disease Development

Unlike the orange rust fungus, the late leaf rust fungus is not systemic. The rust fungus produces two types of spores (urediniospores and teliospores) only on raspberries (Figure 48). The alternate host for the rust is white spruce (Picea canadensis), on which another type of spore (aeciospore) is produced. The rust apparently does not need the aeciospores stage to survive on raspberries, because the disease is found year after year in regions remote from any spruce trees. The fungus probably overwinters on raspberry canes and, in the following season, produces urediniospores that serve as the source of primary inoculum for new infections.

Figure 48: Late leaf rust disease cycle. Taken from the Compendium of Raspberry Diseases and Insects of the American. Phytopathological Society. Used with Permission.

The small, numerous, light-yellow spots seen on the undersurfaces of the leaves are the uredinial pustules that contain the urediniospores of the fungus. These spores are capable of causing new infections throughout the growing season. Black, one-celled teliospores may be found later in the season intermingled with the uredinial pustules. They are capable of infecting the alternate host (spruce) through the production of yet another type of spore (basidiospore), but probably play little part in the life cycle of the rust on Rubus.