Tuesday, October 18, 2011

Today I Learned (TIL)

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.

Fungus on coreopsis

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.


  1. Good information Caroline. We'll know where to come for an id in future.

  2. Love that you share TILs…turning them into TWLs (we). : ) This one is a bit over my head….but I’m sure I’ll be referring back here to help analyze when something looks amiss – nice that you included the links. Cyndi

  3. Good to know...I always wish I knew more about diseases and pests.

  4. I think I have a fungus killing off part of my Sabal minors right now. Did they tell you how to treat it?

  5. Pam, so sorry to hear about your sabals. Dr. Ong spoke at length about the importance of correctly identifying the specific pathogen before applying pesticides. Not every fungicide will kill every type of fungus, and may kill beneficial fungi; if the problem is an insect, virus, bacteria, chemical or climate-related, a fungicide will be completely ineffective.

    Dr. Ong also talked about the disease cycle, emphasizing that even if you've correctly identified the villain and the appropriate treatment, the treatment must be applied at a vulnerable point in the pathogen's life cycle in order to be effective.

    Since I'm a half-baked master gardener intern, I'd suggest calling up the Extension office where a fully trained MG could give you specific recommendations.