This growing season has largely been a bust when it comes to plant diseases. Our dry weather has not been favorable for diseases (particularly leaf diseases) to development. On the other hand, 2023 has been a banner year for insects, and PJ Liesch, the UW insect diagnostician, has been very busy identifying insect problems. As it turns out, plant diseases and insects are not totally unrelated. There are a fair number of disease-causing organisms that can be moved from plant to plant by insects, and it’s these insect-vectored pathogens, and the diseases that they cause, that I have been seeing a lot of this year.
In my June 2021 web article (Fired Up About Fire Blight), I bemoaned the fact that often when people see dying branches on apple, crabapple, and pear trees, they assume the dieback is due to fire blight. Often it is not. But, this year I have seen an uptick of fire blight cases. I suspect that, in part, this is due to increased transmission of the fire blight bacterium (Erwinia amylovora) by honeybees. In the absence of rains during flowering, which tends to discourage their activity, honeybees are out pollinating a wide range of plants, including apples, crabapples, and pears. Honeybees also visit branches where they feed on oozing sap. On trees suffering from fire blight, this sap often contains the fire blight bacterium. Honeybees become contaminated with the pathogen and carry it to apple, crabapple, and pear flowers, where it can infect. As I attempt to diagnose fire blight, I often check dying apple, crabapple, and pear branches to see if infections appear to have started near flower clusters. If so, my fire blight radar goes off.
Among the numerous fire blight cases this year, I had two interesting/sad ones. The first case (more in the interesting category) was fire blight on mountain-ash. Although called an “ash”, this tree (the one with the clusters of bright orange fruits) is in the same plant family as apples, crabapples, and pears, and it is a known host for fire blight. The sample that was submitted to my clinic was from a tree that had numerous dying branches, and it appeared that infections had occurred through flowers. The second case (definitely in the sad category) involved an apple grower who had numerous dwarf trees that he had espaliered. Many of them exhibited dieback, and all of the samples that he submitted tested positive for fire blight. Unfortunately, my recommended pruning for fire blight (i.e., cut 12 inches below where there are obvious symptoms/dieback) functionally meant that the grower had to remove and destroy many of his trees.
I have had a number of vine crop (e.g., cucumber, melon) samples arrive at the clinic with wilting symptoms. Fungal diseases like Verticillium wilt and Fusarium wilt, as well as water mold diseases like Phytophthora root and crown rot, can cause wilting symptoms. However, for many of the cucurbit samples in 2023, not only was there wilting, but there was also significant scarring on the vines from cucumber beetle feeding. These insects can carry Erwinia tracheiphila, the bacterial wilt pathogen, in their intestinal tracts. As they feed, cucumber beetles defecate in their feeding wounds, inoculating plants. The bacterium colonizes the water-conducting tissue of the plants and blocks it off, leading to reduced water movement from the roots to the vines, and thus wilting results.
Diagnosis of bacterial wilt can be relatively easy on certain types of vine crops (particularly cucumber). You cut a wilted vine off near the base of a plant, then cut a roughly six to eight inch vine segment from that severed vine, giving you two vine sections. You then hold the two vine segments apart for about 10 seconds (allowing liquid to ooze from the severed water-conducting tissue), then rub the ends of the cut vine segments together gently for another 10 seconds, then very slowly pull the two vine sections apart. If you see thin, spider web-like strands pull between the two vine pieces, you’re dealing with bacterial wilt. Management of this disease primarily depends on keeping cucumber beetles under control.
There are many viruses that can adversely affect plant growth and productivity. A fair number of these viruses are insect transmitted. Early in the year, when many of my samples come from commercial greenhouses, I tend to see thrips-transmitted viruses. These include Impatiens necrotic spot virus (INSV) and Tomato spotted wilt virus (TSWV). Interestingly, this year I detected TSWV in commercially grown potato plants, where the virus produced necrotic (i.e., dead) spots that were reminiscent of early blight symptoms.
Aphids also commonly transmit plant viruses. In herbaceous ornamentals and vegetables, Cucumber mosaic virus (CMV) is the aphid-transmitted virus that I most commonly see. This virus causes blotchy color (i.e., mosaic) or line patterns on leaves, as well as growth distortions. In recent correspondence with Damon Smith, the UW-Madison field and forage crop pathologist, I learned that he has been seeing another aphid-transmitted virus, Barley yellow dwarf virus (BYDV), in many wheat fields this year. In wheat, BYDV causes yellowing of the upper leaves of the plant, particularly the flag leaf (i.e, the leaf just under the developing grain head). Infection by the virus can lead to significantly reduced yields. BYDV can also infect other “grass” crops including barley, oats, and corn. I had a suspicious corn sample arrive at the clinic in late July, and I suggested the client submit a sample to another lab for BYDV testing (I do not test for this virus at the PDDC).
The diseases described above just scratch the surface of plant diseases where there is a potential insect connection. If you would like read more, check out the UW Plant Disease Facts on ash yellows, aster yellows, Dutch elm disease, ergot, oak wilt, plum pox, soybean vein necrosis disease and thousand cankers disease. After reading, if you if you have questions about these or any other plant diseases, feel free to contact me at firstname.lastname@example.org or (608) 262-2863. I am always here to help.