Vascular plant diseases present a complex series of challenges, both in diagnosis and management. Pathogens of this nature invade the roots and vascular system of their host, stifling the ability of water and essential nutrients to travel throughout the plant. Without proper vascular function, leaves become yellow and wilted. Abnormal discoloration and the pattern which it forms within stem or branch tissue is often the key indication that a vascular disease may be present.
Two of the most prominent vascular pathogens are fungal species of Verticillium or Botryosphaeria. Both infect a wide array of hosts, overlapping to encompass many ornamental trees, shrubs, and vegetable crops. While there are many similarities between the diseases these pathogens cause, there are also defining characteristics which distinguish one from the other.
Verticillium dahliae and verticillium albo-atrum have the potential to cause verticillium wilt. Hosts span from vegetables like tomatoes and potatoes to trees like maple and ash. The fungal structures of either species inhabit soil where infected plant debris has decomposed. There, they can survive for 10-15 years. During the pathogen’s dormant phase, survival structures called microsclerotia germinate. Hyphae (filamentous fungal growth structures) are emitted from these structures and can extend to potential hosts within limited distances. Verticillium inoculates hosts by entering root tips or other areas of root formation, breaking through exterior layers to reach the vascular tissue inside. Conidia (spores) are formed in this location, dispersing to other potential hosts, while Verticillium continues to spread throughout the plant.
Although Verticillium wilt symptoms differ among hosts, abnormalities like leaf yellowing (chlorosis) or browning, wilting, and branch dieback raise suspicion of infection across the board. Symptoms may be isolated to one side of the plant or individual branches. Branch or stem discoloration is another key indicator of infection, accompanied by a streaking and a characteristic ring-shaped brown pattern on the interior of woody tissue. Damage is often concentrated in the lower areas of the plant, as Verticillium invades hosts from their base, working upwards.
The persistence and longevity of Verticillium structures in soil makes crop rotation and the destruction of diseased tissue crucial to disease management. While it’s usually best to remove the diseased plant entirely, woody plants with minor infections can survive for several more years, provided affected branches are pruned away and tools used for pruning are disinfected after use. Rubbing alcohol is a good choice, but there are several other disinfectants on the market labeled for use with tools. Do not compost or use diseased tissue for wood chips, as remaining fungal structures will survive for several years and may infect new host plants.
Botryosphaeria canker and dieback are attributed to several causal agents in the Botryosphaeriaceae family. Botryosphaeria is an opportunistic pathogen, choosing already stressed plants to infect and taking advantage of their weakened defense systems. Stress can stem from many sources—frequently environmental challenges, insect infestations, or prior instances of disease. While this group of pathogens has an extensive range of hosts, it is often observed on woody ornamentals—viburnum or rhododendron, for example—and trees like dogwood or maple. Fungal structures overwinter on previously infected tissue, or as endophytes which silently colonize plants without causing symptoms until they become weakened. Botryosphaeria uses many routes of entry when invading its hosts, including open wounds, cracks, or lenticels—porous openings on branch and stem tissue used for gas exchange.
As a vascular pathogen, wilting and dieback are characteristic symptoms of disease, accompanied by necrotic patches on bark and branches. Branches become girdled, and sunken cankers are evident. Depending on the host, sap may ooze from these areas, and black fungal sporulation (pycnidia) may be visible. Unlike verticillium wilt, interior vascular discoloration due to Botryosphaeria infection appears in wedges, resembling a slice of pie.
To manage Botryosphaeria canker and dieback, it is important to minimize sources of stress which can leave plants susceptible to infection. Appropriate pruning and sanitation practices, routine fertilization in accordance with the plant’s needs, and protection from environmental stressors (drought, heat, or cold) are all vital to maintaining vigor. Take care to prevent mechanical injury and wounds, minimizing the potential modes of entry for the pathogen.
As is the case with all plant diseases, laboratory analysis is required for an accurate diagnosis. If you suspect your plant has been infected by a vascular disease, submit branch tissue to the UConn Plant Diagnostic Laboratory for evaluation. Remember to collect tissue that is just beginning to appear symptomatic, as the lab is unable to diagnose plants that are already dead.
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