Each month, the Plant Disease and Insect Diagnostic Laboratory (PDIDL) provides a summary of plant health problems from Oklahoma specialty crop growers (horticultural crops). The volume of samples submitted to the lab has increased with the warmer weather. A summary of sample submissions is shown in Table 1.

Bold indicates carry over from the previous month, or into the next month's report.

In March, the PDIDL received several image samples of junipers, arborvitae and pines with wilting and/or browning foliage. A few physical specimens were submitted to the lab for examination. The main problem with these evergreens is environmental stress, generally drought from low soil moisture and drying winter winds. Cold temperature injury may also be involved with the needle browning.

Figure 1. Pine shoot with needle tip browning caused by environmental stress factors (mainly drought and wind).

Of special interest this month was a photo submitted by Arborscapes Tree Service Inc. of a river birch showing an orange slime on the trunk of the tree (Figure 2). Based on the color of the exudate, it is most likely Fusicolla merismoides (previously known as Fusarium merismoides). This fungus, as well as other fungi and yeasts, will colonize sap that is leaking from a tree wound. As it appears to be developing along a long, vertical crack, this tree most likely suffered an injury, perhaps a frost or freeze crack from cold temperatures this winter. Alternatively, it may have occurred last year due to heat, drought, or some other unknown factor.

Figure 2. River birch with orange substance on the trunk. Image provided by Arborscapes Tree Services, Inc.

Why is this orange exudate present and is it a problem?
In the spring, tree roots begin actively moving water from the roots to the shoots to support the developing canopy. When there is increased sap-flow, if there is a crack in the trunk, some of the sap may leak from the site. Fungi (such as F. merismoides) can get nutrition from the exudate and will live at the leaking site. Many pathologists believe that this fungus is not harmful to the tree, and it is just colonizing the exudate. Others believe it can act as a weak or opportunistic pathogen. Regardless, the crack in the trunk is the more significant problem. Sometimes, vertical cracks will repair themselves as the tree trunk grows and enlarges (it grows back together). In other situations, tree health is negatively affected, and the crack leads to more significant problems. The suggestion is to provide good plant care, especially irrigation during periods of drought, and follow a "wait and see" approach.

In the PDIDL, we often identify fungi by examining fungal structures with dissecting and compound microscopes. Recently, we used this process to show a graduate student in the OSU Department of Entomology and Plant Pathology, Luana Muller, an interesting phenomenon that one fungus displays under certain conditions. The organism viewed, Phytophthora, produces sporangia, and although these spore structures can germinate directly (like a seed germinates), sometimes the sporangia give rise to motile zoospores. You can see this process in this YouTube Shorts video filmed by Ms. Muller. The sporangia (often lemon shaped) will open at one end, forming a goblet-like structure, and small circular motile spores will leave the structure through the opening. These zoospores move in films of water to initiate new infections in susceptible plants. This is why Phytophthora-diseases are often most severe in heavy, overwatered soils or under damp conditions.

Look for additional plant health updates from the PDIDL throughout 2025. If you have suggestions for future topics, please contact jen.olson@okstate.edu or call the PDIDL at (405) 744-9961.