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Cane diseases: Spur blight, cane blight and anthracnose

There are three cane diseases in Minnesota that can reduce or destroy a crop in summer-bearing raspberries. For red raspberries, the most common cane diseases are cane blight (Leptosphaeria coniothyrium) and spur blight (Didyimella applanata). For black raspberries, the most common cane disease is anthracnose (Elsinoe venata). Anthracnose also can infect red raspberries but is less common.

Crop losses due to cane diseases

In fall-bearing raspberries, spur blight is primarily a leaf disease. Older leaves near the ground are the first to be infected and die. When only the older leaves in the bottom third of the cane are killed, crop losses are minor. During severe outbreaks spur blight will kill younger leaves towards the top of the cane, resulting in reduced vigor, shorter canes and reduced yields.

In summer-bearing red raspberries, spur blight is a cane disease that can weaken or kill canes that have overwintered. In raspberry patches that have not been properly pruned, spur blight and cane blight can kill over 90% of the canes to the ground following summers with high disease pressure.

Severe anthracnose weakens small black or red raspberry canes, by killing parts of the bark and causing large cracks in the bark. Anthracnose makes the canes more susceptible to winter injury, and canes with anthracnose often die to snow level. In heavy snow years, there may be little winter injury in infected canes, while in low snow years infected canes may die to a few inches above the ground.

Identification

The best time to identify all three cane diseases is to look at primocanes in late summer and early fall. Primocanes are first year canes that sprout in the spring. In summer, the primocanes have green bark, which is susceptible to infection by the fungi that cause cane diseases. The distinctive symptoms of each disease can be seen in early fall before the bark turns brown. In the spring, the overwintering canes are often dead from the disease, and diseased canes are often mistaken for winter injury. Disease symptoms are not easy to distinguish in brown bark and dead canes. Each disease has specific symptoms, and each infects a different part of the cane.

Spur blight

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Spur blight on the leaves of 'Autumn Britten' red raspberry. The edges of the leaves in the middle of the cane are dead, and the leaf stems on the lower leaves are still attached after the leaf blade fell off .

Thaddeus McCamant, Central Lakes College

Spur blight is both a leaf disease and a cane disease. Infections start out in the leaves, causing the edges of the leaves to turn yellow and die. Lower leaves are most likely to be infected, and the damage can be mistaken for normal leaf senescence. When leaves are killed by spur blight, the petioles (leaf stem) remains on the cane even after the leaf falls off, whereas when leaves senesce, the entire leaf falls off. The spur blight fungus moves from infected leaves into the cane.

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Spur blight infection at the node of 'Nova' raspberry cane. The infection started in the leaf and spread into the cane.

Thaddeus McCamant, Central Lakes College

In the canes, spur blight starts out as an indistinct chocolate brown or purple spot just below the point where a leaf was attached to the primocane. The lesions start out about 1/2 inch in diameter, but quickly grow, sometimes encircling the entire cane. These lesions are easily seen in primocanes, but may not be noticeable the following year when the canes develop brown bark.

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Peeling bark on 'Killarney' floricanes that had spur blight the previous summer

Thaddeus McCamant, Central Lakes College

In overwintered canes, buds next to the infected nodes usually do not sprout in the spring, causing the plants to be "leggy", with large areas of the lower cane producing no leaves or flowers. In the spring, the bark peels away from the cane in floricanes. When looking at the peeled bark with a magnifying glass, little black dots are visible. The little black dots are the spore producing structures of the spur blight fungus that will infect primocanes the next summer.

In the spring, spur blight is often confused with winter injury. When winter injury is the only cause of cane death, the canes die to snow level, and the floricanes will sprout in living buds below the snow level (see abiotic diseases). In raspberry patches infected with spur blight, the floricanes die to the ground. With spur blight, small canes that sprouted later in the summer are more likely to die than large canes, and canes inside the row are more likely to die than canes on the edge of the row. With winter injury, dieback is fairly even across a row.

Cane blight

Cane blight infections start in wound sites of the primocanes. A wound site can be where the primocanes were tipped, or where canes rubbed against each other or against a trellis wire. The infection spreads through the cane from the wound and causes cankers to form. Cankers caused by the cane blight fungus start out as reddish- brown streaks under the bark. The cankers can span several inches up and down the cane and may girdle the cane. Leaves arising from the infected section of the stem may wilt and die. If the infection girdles the stem, the entire cane may wilt and die.

Anthracnose

cane-diseases

Jewel black raspberry internode with anthracnose lesions.

Thaddeus McCamant, Central Lakes College

cane-diseases

Anthracnose on a red raspberry primocane.

Michelle Grabowski, UMN Extension

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Anthracnose in leaves of a red raspberry plant.

Michelle Grabowski, UMN Extension

Anthracnose can be identified by little round, sunken pits in the bark of the cane. Quite often the margins of the lesions are slightly raised above the surrounding bark. The spots are white to pale tan, while the margins sometimes are a purplish red. Anthracnose spots tend to be less than 1/4 inch in diameter, which is smaller than spur blight or cane blight. Unlike spur blight, the anthracnose lesions are scattered throughout the cane between the nodes . Anthracnose is very common on black raspberries but it can also occur on susceptible red raspberry varieties. When disease pressure is high, the leaves will also have small round purple spots with a light colored center. In the winter, raspberry canes with anthracnose often die to snow level.

Important biology

All three cane diseases spread from the floricanes to the primocanes in early summer. None of the cane diseases infect the roots, and the new primocanes must be infected each summer for the disease to continue. The fungi that cause anthracnose and cane blight can only be spread by splashing water, while the fungus that causes spur blight can spread by the wind as well as splashing water. Fungal spores of all three diseases are produced on infected floricanes and spread when irrigation water or rain splashes spores to new primocanes. Once the primocanes are infected, splashing water can move spores throughout the plant.

Anthracnose can be introduced to a raspberry patch on newly purchased raspberry plants that are infected with the disease or from nearby, wild plants. In black raspberries, the disease spreads readily from infected floricanes to new primocanes that sprout next to the mother plants. Because anthracnose is primarily spread by water, new plants that sprout by tip rooting away from the mother plant can escape infection.

Since spur blight spores travel through the air, the disease is found in nearly every raspberry patch in Minnesota. Fungal spores land on the leaves and germinate and infect leaves during warm, wet spells. Young leaves are fairly resistant to the fungus, and most infections start in leaves on the lower third of the cane. The infection spreads from the leaf to the stem and from the stem to the cane. The infection then spreads both up and down the cane. Spur blight epidemics are most common in years with frequent rain during the month of June.

Management

All three cane diseases thrive in moist conditions. Improving air flow through the patch will allow plants to dry quickly after rain or dew. Use drip irrigation when possible. If sprinkler irrigation is the only option, apply water early on a sunny day so that leaves dry quickly in the sun. Keeping the rows narrow will help the plants dry quickly. A recommended row width is 18 inches at the soil. Mow canes that have spread into the walking aisle to maintain a narrow planting row. Remove all weeds from the raspberry patch to improve air circulation around the canes.

Always remove floricanes after they have produced a crop. Typically, old floricanes start dying towards the end of harvest, but some will remain green until early fall. For disease control, the best time to remove dead canes is during the weeks following harvest in late July or early August, even if some of the leaves are still green. At this time, any primocanes showing clear disease symptoms should also be removed from the patch. Cutting dead and infected canes removes the fungal pathogen from the patch and increases air flow through the raspberry patch so that the canes can dry quickly after a rainfall. Be sure not to discard diseased canes in the immediate vicinity of the raspberry patch as they will be a source of spores for new infections.

A more drastic way to control spur blight and cane blight is to mow the whole raspberry patch down in late winter or early spring and remove or burn the canes. Mowing will reduce cane diseases and protect the crop for the following summer. All cane diseases move from the overwintered floricanes to the newly-sprouted primocanes. Cutting and removing all floricanes will sharply reduce new infections. Mowing is often used for fall-bearing raspberries to reduce labor. Fall-bearing raspberries will produce fruit on primocanes and therefore will still produce a crop after the patch is mowed. Mowing can be done for summer-bearing raspberries if spur blight or cane blight has been a significant problem. If all the canes are removed from a patch of summer-bearing raspberries, however, the patch will not produce fruit that summer. In some years, spur blight kills nearly all of the canes in certain raspberry patches, resulting in little or no crop the following summer. Inspect the patch after the leaves sprout in the spring. If most of the canes died to the ground during the winter, removing all the canes will help protect the plants in future years.

Fungicides are rarely necessary, but they can be used to reduce cane diseases in severely infected patches. The best time to spray for cane diseases is in early summer, before a wet spell allows the primocanes to become infected. Fungicides with the active ingredients copper sulfate, copper sulfur or myclobutanil provide some protection from cane diseases in raspberries. Actinovate and Oxidate also provide some control and are approved for organic production. All fungicides should be aimed at the primocanes. Fungicides alone provide only partial disease control and are more effective at controlling cane diseases if the canes have been mowed in late winter.


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