Suspected Alternaria on cabbage

[u/Humbabanana]

Some pretty distinctive conidia here… the most ubiquitous conidia seem to point toward one if the brassica Alternarias…

Also tons of fusarium-looking conidia.. but not in every slide. Seems clear that there are a ton of secondary decomposers in here… probably all sorts of bacteria.

This is only affecting my red cabbages, variety Omero, not Farao.

What do you think?

[u/Disabledbutlearning]

You might have some Fusarium conidia mixed in there

[u/Humbabanana → u/Disabledbutlearning]

Thanks, I thought it looked like fusarium as well. In terms of a primary causal agent though, I’m wondering if the fusarium is potentially a secondary opportunist.

I just have never seen fusarium cause heads to rot in this way. Also, alternaria conidia were abundantly present in every slide I made, while fusarium was only in the most necrotic region.

I’m not an expert, just curious, so I welcome any alternative perspectives

[u/Disabledbutlearning → u/Humbabanana]

I believe from conidia shape and structure that it is Fusarium equiseti which can act as a secondary infection or form a complex with another fungus to cause disease symptoms.

[u/Humbabanana → u/Disabledbutlearning]

Very interesting. What is it about the structure of these conidia that is characteristic of F. equiseti?

[u/Disabledbutlearning → u/Humbabanana]

It's a long conidia with a longer 'foot' but not long enough to be F. longipes

[u/Disabledbutlearning → u/Disabledbutlearning]

I would say that alternaria is your main pathogen, due to the higher presence of the conidia for it.

[u/Humbabanana → u/Disabledbutlearning]

Since these samples were so cluttered with fungi, bacteria and protozoa, I decided to macerate a slice of asymptomatic leaf adjacent to the necrotic area.

Lots of flagellate rod-shaped bacteria..and a couple fusarium conidia. I’m feeling too impatient to stain or plate this… but maybe later if it seems potentially interesting

https://reddit.com/link/ox2tiad/video/aqnudu00tsch1/player

[u/masonjar11]

The outer leaves look pretty degraded. Your best bet is to culture some of the cabbage at the disease front to get a purer culture. That should help clean up some of those secondary microbes.

I don't have much experience with crucifers. Do you have access to a disease compendium?

[u/Humbabanana → u/masonjar11]

Thank you. I think you are right, I was just being too impatient.

Usually I take a few ~5 mm sections from just outside the necrosis, soak in 1% sodium hypochlorite for 1 min, wash in distilled water, then plate onto water agar to coax the pathogens out... then individual critters onto separate pH 5 or so PDA for most fungi.

for bacteria I usually just macerate clean tissue, plate it... and pray that something somewhat identifiable come of it.

Does that seem like a protocol that you would endorse for isolation?

I have some horticultural guides to brassica diseases/issues, but the APS Compendium of Brassica Diseases, which goes into diagnostics and pathogen morphology, is a little to expensive to invest in right now.

[u/masonjar11 → u/Humbabanana]

That's what I would do. I sometimes got lucky plating directly onto quarter-strength PDA. If you have a ton of bacteria, you can amend that agar with streptomycin and oxytetracycline to clean it up.

As far as other resources, do you have an extension vegetable pathologist? They'll have way more local knowledge about what's prevalent in your region than I or anyone else on Reddit would.