Bramble Root Diseases

Phytophthora Root Rot

Phytophthora root rot is caused by several related species of soilborne fungi belonging to the genus Phytophthora. To date, P. megasperma, P. cryptogea, P. citriocola, P. cactorum, and at least two additional unidentified Phytophthora species have been implicated in this disease. The disease occurs on red, black, and purple raspberries, although in the northeastern United States it has been documented most commonly on red raspberries. The disease has reported to occur in blackberries in Kentucky. Phytophthora root rot can be an extremely destructive disease on susceptible cultivars where conditions favor its development. Infected plants become weak and stunted and are particularly susceptible to winter injury; seriously infected plants commonly collapse and die. Key methods of control include site selection or improvement to avoid saturated soils and the selection of more resistant red raspberry varieties.

Symptoms

The disease is most commonly associated with heavy soils or portions of the planting that are the slowest to drain (lower ends of rows, dips in the field, etc.). In fact, most declining plants that are considered suffering from "Wet feet" probably are suffering from Phytophthora root rot. Symptoms include a general lack of vigor and a sparse plant stand. Apparently healthy canes may suddenly decline and collapse during the late spring or summer (Figure 50). In such cases, leaves may initially take on a yellow, red, or orange color or appear scorched along the edges. As the disease progresses, affected canes wilt and die. Infected plants frequently occur in patches, which may spread along the row if conditions remain favorable for disease development.

Figure 50: Above ground symptoms of Phytophthora root rot on primocanes of 'Heritage' red raspberry.

Because wilting and collapsing may be caused by other factors (winter injury, cane borers, etc.), it is necessary to examine the root system of infected plants to diagnose the disease. Suspect plants should be dug up and the epidermis (outer surface) scraped off the main roots and crown. On healthy plants, the tissue just beneath the epidermis should be white; on plants with Phytophthora root rot, this tissue will be a characteristic brick red (eventually turning dark brown as the tissue decays) (Figure 51). Sometimes a distinct line can be seen between infected and healthy tissue, especially on the below-ground portion of the crown.

Figure 51: Below ground symptoms of Phytophthora root and crown rot on red raspberry. Note the sharp line of demarcation between healthy, white tissue and infected reddish-brown tissue. This reddish-brown or brick-red discoloration on roots is typical of Phytophthora root rot.

Disease Development

The fungi persist primarily as mycelium in infected roots or as dormant resting spores in the soil. When the soil is moist, reproductive structures (sporangia) are formed upon the infected tissue or by germinating resting spores (oospores) in the soil. Within each of these structures a number of individual spores called zoospores are formed. These zoospores are expelled into the soil during periods when the soil is saturated with water. The zoospores have 'Tails' (flagella), which allow them to swim through the water-filled soil pores to reach new plant parts. Upon reaching a plant root or crown, the zoospores become attached and infect. As water remains standing and oxygen is depleted from the root zone, the plant is progressively less capable of resisting the fungus and infection becomes more likely and severe. Each new infection site is a potential source of additional resting spores and zoospores, allowing for epidemic disease development in sites which are subjected to repeated periods of standing water. Although the optimum season for infection is not known for certain, it is likely that spring and fall are particularly favorable periods. However, it is assumed that infection can occur throughout the growing season if soil moisture conditions are favorable.

Verticillium Wilt

Verticillium wilt is caused by the soilborne fungus Verticillium dahliae and is one of the most serious diseases of raspberries. This disease reduces raspberry yields by wilting, stunting, and eventually killing the fruiting cane or the entire plant. The disease is usually more severe in black and purple than in red raspberries. Blackberries are also susceptible to the disease, but seldom suffer severe losses.

Verticillium wilt is usually a cool-weather disease and is most severe in poorly drained soils and following cold, wet springs. The appearance of symptoms on new canes frequently coincides with drought stress during hot, dry, midsummer weather. Key methods for control are site selection and proper crop rotation to avoid planting in infested soils.

Symptoms

Symptoms usually appear on black raspberries in June to early July, and on red raspberries about a month later. The lower leaves of diseased plants may at first appear to have a dull green cast as compared to the bright green of normal leaves. Starting at the base of the cane and progressing upward, leaves wilt, turn yellow, and drop. Eventually, the cane may be completely defoliated except for a few leaves at the top (Figure 52). Black raspberry and blackberry canes may exhibit a blue or purple streak from the soil line extending up the cane to varying heights (Figure 53). This streak is often not present or is difficult to detect on red raspberries. In the spring following infection, many of the diseased canes are dead. Others are poorly developed and have shriveled buds. The new leaves are usually yellow and stunted. Infected canes may die before fruit matures, resulting in withered, small, and tasteless berries.

Figure 52: Verticillium wilt symptoms on black raspberry plant.

Figure 53: Blueish streaks can often be observed on black raspberry canes affected by verticillium wilt.

Disease Development

Verticillium is a common soilborne fungus. It causes disease on more than 160 different kinds of plants, including strawberries, eggplant, tomatoes, potatoes, stone fruits and peppers. The fungus overwinters in the soil and plant debris as dormant mycelium or tiny black specks called microsclerotia. The fungus can survive in the soil for many years. When conditions are favorable, microsclerotia germinate and produce threadlike fungal filaments (hyphae). These hyphae can penetrate the root directly, but invasion is aided by breaks or wounds in the roots. Once inside the root, the fungus grows into the water-conducting tissue (xylem). The destruction of water-conducting tissue prevents the movement of water from the roots to the rest of the plant. Thus, the plant eventually wilts and dies.

Bacterial Crown Gall and Cane Gall

Crown gall is caused by the bacterium Agrobacterium tumefaciens. Cane gall is caused by a very similar bacterium, Agrobacterium rubi. Crown gall is a widespread disease of all brambles. Cane gall affects black and purple raspberries more frequently than red raspberries or blackberries. These diseases are particularly serious in nursery fields where freedom from the disease is essential. The bacteria induce galls or tumors on the roots, crowns, or canes of infected plants. Galls interfere with water and nutrient flow in the plants. Seriously infected plants may become weakened, stunted, and unproductive. Key methods of control include starting the planting with disease free plants and crop rotation. A biocontrol agent (Galltrol) is currently available as a preplant treatment.

Symptoms

Young galls (tumorlike swellings) are rough, spongy, and wart-like (Figure 54). Galls can be formed each season and vary in size from a pinhead to several inches in diameter. They develop near the soil line or underground in the spring. Cane galls occur almost exclusively on fruiting canes and usually appear in late spring or early summer. Both crown and cane galls become hard, brown to black, woody knots as they age. Some disintegrate with time and others may remain for the life of the plant. The tops of infected plants may show no symptoms, but plants with numerous galls may be stunted; produce dry, poorly-developed berries; break easily and fall over; or show various deficiency symptoms due to impaired uptake and transport of nutrients and water.

Figure 54: Gall on the root of a crown gall infected red raspberry plant.

Disease Development

Crown gall bacteria enter the plant only through natural openings or wounds in the epidermis or bark of the plant. The bacteria survive in infested soil for years and can invade the roots and crowns of susceptible plants through natural growth cracks, tissue damaged by winter injury, or damage caused by soil insects. Man-made wounds that occur during pruning and cultivation are important points of entry. After the bacteria enter plant tissues, an incubation period of 11 to 28 days, or more if the host is dormant, may be required before the bacteria induce cell proliferation, enlargement, and disorganized growth, resulting in the production of galls. Bacteria, abundant in the outer portions of galls, are continually sloughed off into the soil. The bacteria overwinter in soil and in diseased galls. The following spring, these bacteria are spread by splashing rain, water, cultivation (any practice that moves soil), pruning tools and insect feeding. When they contact wounded tissue of a susceptible host, they enter and induce gall formation, completing the disease cycle.