Soilborne fungus can afflict hebes and a wide range of other host plants

Hebes (pronounced heebees) are an interesting group of evergreen shrubs, mainly from New Zealand, although there are also a few from South America and French Polynesia. Botanically, they are now placed in the genus Veronica, but many will know them by their older genus name, Hebe.

Hebes are well loved by plant aficionados for their symmetrically arranged leaves, interesting growth forms, and flowers. The leaves come in various shades of green, blue, bronze, or purple, or they can also be variegated with white.

Species and cultivars with smaller leaves are generally more hardy and do well in western Oregon and Washington. Those with larger leaves, however, are more sensitive to cold and can only be successfully grown as perennials along the coast or other locations where the winter temperatures are milder.

Over the past five years, I have seen an increasing number of landscape hebes die during the summer months. Symptoms first become noticeable when the leaves on one or two stems per plant begin to turn yellow and wilt. As the symptoms progress, the tips of the branches droop and eventually the entire branch dies, turning brown and brittle.

On larger plants, the disease may progress slowly, sporadically taking out a branch or two over several years. At least one Sutherland hebe (pictured) appears to have recovered and has had no new symptoms for five years. On smaller, younger plants (less than 3 years old), however, the disease is quite rapid and the entire plant appears to dry up and die within a few weeks.

Symptoms are occasionally noticeable as early as February, but are more commonly observed once the weather warms up later in spring and summer. Affected species have included the cypress hebe (V. cupressoides), thick-leaved hebe (V. pinguifolia), and whipcord hebe (V. ochracea).

Finding the real culprit

Suspecting Phytophthora root rot, I dug up plants from several locations and examined the root system. The roots were mostly intact, with more fine roots than I would expect had Phytophthora been the culprit.

Phytophthora species tend to rot away the fine roots, leaving thicker, coarse structural roots behind. Roots infected by Phytophthora also tend to be water soaked or mushy. Instead, the hebe roots were dry and hard, with some of them turning grey or black.

Curious, I took samples back to the lab, where I was able to isolate the pathogen that causes charcoal rot, Macrophomina phaseolina.

Charcoal rot is considered a warm weather disease. The pathogen is soilborne and becomes active once soil temperatures are above 60°F. However, it is most active at warmer temperatures between 82-90°F.

Unlike Phytophthora species, which are water molds and require free moisture for infection, the M. phaseolina fungus is able to infect and cause damage under relatively dry conditions. Disease may actually be worse in sandy, well-drained soils. As a result, charcoal rot symptoms often intensify during hot summer weather and are exacerbated by drought.

The fungus can survive for years in the soil as tiny, black microsclerotia. When roots of a host plant grow nearby, the microsclerotia germinate, and then infect and kill the roots. Infected roots quickly lose their ability to transport water and nutrients. This leads to aboveground symptoms of nutrient deficiency (yellowing or stunting) and water stress (wilting).

Root and stem tissues near the root collar may become discolored internally, turning orange or brown with prominent gray or black streaking. The root exterior may also turn black as new microsclerotia are produced both internally and on the roots surface. It’s this black color on the infected tissues that gives rise to the common name for the disease, charcoal rot.

The fungus has an extremely wide host range and can infect well over 500 plant species, including vegetables (beets, cucumber, peas, peppers), flowering annuals and perennials (dahlia, lavender, sunflower), conifers (cedar, Douglas-fir, pine, spruce), monocots (lily, palms), small fruits (blueberry, grape, and strawberry) and deciduous trees and shrubs (cherry, dogwood, maple, oak).

Probably one of most well-known hosts of this pathogen is strawberry, where infection results in discrete patches of wilted or dying plants in the field. Charcoal rot is also a significant problem in conifer seedling beds, where it can cause damping-off.

In most of these hosts, the symptoms of charcoal rot can be easily confused with those caused by other root pathogens, such as Fusarium, Phytophthora, and Verticillium. Therefore, a professional diagnosis should always be made before any disease control treatments are applied.

Established in the Willamette Valley

Up to this point, charcoal rot has mainly been a problem in more southern states, where the soil temperatures are much warmer in the summer and the winters are more mild. But, lately, the pathogen has been discovered with increasing frequency further north. In 2014, M. phaseolina was first detected by the Oregon State University Plant Clinic on strawberry in southern Oregon.

Looking back through my lab notes, I see I have also isolated M. phaseolina from nursery crops in the Willamette Valley, including several declining red maples at a tree nursery in 2010 and from a few chlorotic and stunted field-grown rhododendrons at another nursery in 2017.

So how did a pathogen that was primarily restricted to more southerly locations and requiring warmer temperatures start appearing further north in the Willamette Valley? One of the most likely routes is via plant trade. Nine of the younger hebes that developed charcoal rot originated from a single nursery.

Hebes are often produced in greenhouses, which are more sheltered and have warmer growing conditions than outdoors. They also prefer to be grown on the drier side, disliking excessive wet. These same warm and dry conditions would be equally suitable for M. phaseolina to thrive if it had been accidentally brought into a greenhouse production system on contaminated media.

In addition, the climate in the Pacific Northwest has become more favorable for this M. phaseolina over time. Between last year’s heat dome and the record drought, and this year’s week-long heatwave in late July, it appears that hot and dry summers are becoming more commonplace. This disease is likely to become more prevalent in our region as long as these trends continue.

Managing the disease

Prevention and sanitation are key for managing charcoal rot. The most likely avenues for getting this disease are by accidentally bringing it in on infected plant material from another nursery or through root contact with contaminated potting media, field soil, or dirty containers.

Be sure to inspect all new, incoming plants for symptoms. Reject the shipment if you see anything suspicious. In addition, keep all container plants off of the ground, ideally on raised benches with wire mesh bottoms to prevent roots from coming into contact with soil.

If you do see symptoms of charcoal rot on your own nursery stock, send samples to a plant disease diagnostic lab for confirmation. As mentioned, there are several root pathogens that cause similar symptoms, and treatments will vary depending upon which pathogen is causing the disease.

That said, there are no treatments that can cure a plant once infection has occurred. Remove and destroy all infected plants, including the roots and any adhering media or soil to reduce the amount of inoculum that could get left behind.

For container production, always use new potting media and new, or thoroughly cleaned and disinfested, pots. Never re-use potting media from plants that have died.

Composting and solarization are unlikely to be very effective because of how tolerant M. phaseolina is to heat, but fumigation or anaerobic soil disinfestation may be effective at reducing microsclerotia populations in infested field soil.

Dr. Jerry E. Weiland is a research plant pathologist with the Horticultural Crops Disease and Pest Management Research Unit, USDA – Agricultural Research Service in Corvallis, Oregon. He can be reached at [email protected].

References

Sinclair, W.A. and Lyon, H.H. 2005. Diseases of Trees and Shrubs, 2nd ed. Cornell University Press, Ithaca, NY.

Pscheidt, J.W. and Ocamb, C.M., eds. 2022. Pacific Northwest Plant Disease Management Handbook. Oregon State University, Corvallis, OR.

Weiland, J.E. 2022. First report of Macrophomina phaseolina causing charcoal root rot of hebe (Veronica cupressoides, V. ochracea, and V. pinguifolia). Plant Disease 106: 1984.