Today I learned how to tell the difference between bacterial plant infections and fungal plant infections.
During our 6 hour class on plant pathology, Dr. Kevin Ong, Director of the Texas Plant Disease Diagnostic Laboratory, explained that both bacteria and fungi can cause blight, wilt and leaf spots, but there are some visible differences between the two, if you can catch the disease early on.
Fungal leaf spots tend to spread across the veins of leaves, while bacterial leaf spots start off as angular-shaped lesions confined within leaf veins. Other clues: spore accumulations may be visible in fungal infections, while bacterial infections more commonly have a wet appearance. And if a cut section of a leaf or stem is placed in water, a bacteria-infested plant is more likely to ooze a gummy substance than a plant suffering from a fungal infection.
I wish I'd known this a year and a half ago, when a mystery disease struck and killed an established Salvia greggii, a Mayfield Giant coreopsis and a Pringle's Bee Balm. Dr. Ong says that fungi are responsible for 85% of common garden diseases, so when the culprit's in doubt, it's probably a fungus. Looking back at the photos, I think my suspicion of a fungus was probably correct. The leaf spots on my coreopsis spread across the leaf veins and I could see what looked like visible black spores on the leaves. I detected no angular leaf spots confined by the leaf veins, and the spots didn't look wet. But if such a calamity were to happen now, I'd cut off a stem or leaf, stick it in water and look for ooze, and I'd dig up the roots to look for root rot or nematodes. I might even send off a specimen to be tested for $35.
And the white growth I saw underneath the Bath's dianthus that spring? Probably benign. Dr. Ong says that of the 100,000 known fungi, only 8% are pathogenic; the other 92% are innocent bystanders.
For more information on the Texas Plant Disease Diagnostic Lab, visit their website or their Facebook page.
(Today I Learned features a nugget of information I learned during training to become a Travis County Master Gardener.)
Words and photos © 2009-2011 Caroline Homer for "The Shovel-Ready Garden". Unauthorized reproduction is prohibited.