Leaf Necrosis

cmcramer · 2025-09-13T17:08:09+00:00

Appears in late season (started a few weeks ago) , starts on bottom leaves and progresses up, most of plant still looks OK to me.....top leaves maybe a littl...

Teddahlia

My theory: I am always suspicious of yellow on plants when there is a lot of compost or organic material covering the soil below the plants. Bacteria use the available fertilizer to digest organic material and the plants can become nutrient starved. Quickest solution: Apply some liquid fertilizer. Downside: It is late in the season to be fertilizing.

cmcramer

Teddahlia Interesting idea Ted. Maybe next year I'll experiment. Mid season nitrogen has always scared me off. Guess I could not mulch a row and see what happens....but that mulch sure helps with moisture retention.

cmcramer

I should add that plants were given two Tablespoons of Osmocote Bloom with micronutrients, NPK 12-7-18, 2 to 3 month release (contains Mg, S, B, Fe, Mn, Mo) in early June.

Teddahlia

Time release fertilizer is released at diminished rate near the end of it's life. We add a "booster' dose of fertilizer after about 6 weeks. Farmers use the color of the leaves to measure how much fertilizer to add and even have leaf meters to check on nutrients.

Teddahlia

In Australia they like to use alfalfa hay as mulch as it holds in moisture and breaks down into fertilizer.

cmcramer

I use oat straw mostly because it is easily available to me. Since I have three Miss Molly plants....one was just volunteered for a very late season experiment. I sprinkled in 2 Tablespoons of Ammonium sulfate and watered in well. Will keep up the water to dissolve and move the fertilizer down to roots. Can't hurt at this point!

Bessie

cmcramer it would be great to hear an update to your Miss Molly fertilizer experiment.

SteveM

cmcramer Does the leaf close-up show interveinal chlorosis?

I don't see any interveinal chlorosis.

I always have to remember to cut down on the watering frequency in the Fall when the temperature cools and the plant growth slows. My first guess is that this plant is getting too much water. My second guess is a lack of nitrogen.
I would first check the soil moisture at tuber/root level to make sure this plant is not in an area that is getting too much water. If it is a lack of nitrogen, a soil drench of 1 Tablespoon of Miracle Grow/ gallon of water will green it up in about a week.

cmcramer

SteveM Thanks Steve. Can't be too wet - we are in a drought up here in NY and I water only every 6/7 days because I'm on a well - country livin'. But lack of N could be....

I'l try your Miracle Grow test on an 'exttra' plant!

Teddahlia

Ammonium Sulfate is on my "do not use" list as it is extremely acidic, more so than just about any fertilizer. Our soil is already too acidic. In parts of the USA , where soil is too alkaline, it is one of the best fertilizers. Historically, nitrogen fertilizers were nitrates. Nowadays, they seldom put nitrates in fertilizer for a couple of reasons. Of course cost is number one, too expensive. But number two is they are used as explosives. A nurseryman once told me his favorite fertilizer was calcium nitrate. It turns plants green in a few days.

Bessie

cmcramer if you're interested, human urine used as a fertilizer will make your dahlias look like they've been on a vacation and gotten a spa treatment😅. They will glow and look healthy.
Dilute with water at about 1:5 ratio, or whatever seems right to you.

cmcramer

UREA! .... I mean EUREKA !...

Bessie

cmcramer I'm curious how the plant is doing that you showed in your original photo that showed the yellowing. Did the buds bloom? What actions did you take, if any.

cmcramer

On one plant I 'drenched' its soil with Miraclegrow, per Steve's advice. To a second plant I sprinkled one tablespoon Ammonium Sulphate and watered heavily, repeatedly. No change in either plant. Both are flowering just fine. If this yellowing is due to the heavy straw mulch/microbial action consuming Nitrogen....I may just have to ignore it in the future, because heavy mulching is required when water comes from a country well! Or I could experiement with adding a mid-July nitrogen dose; something I have not done for fear of over fertilization/over vegetative growth.

Teddahlia

Fertilizer is a three bears product: (1) Too much is bad(2) too little is just as bad (3) Just right means some all the time during the growth cycle. My favorite all time saying is "too much of a good thing is a bad thing." And the converse of of that is "too little of a good thing is a bad thing". I believe in your case using mulch that uses soil bacteria that feed on nitrogen, makes the nitrogen unavailable to the plants. The best mulch in my opinion(don't trust my opinions) is mint compost. It has enough nitrogen in it to satisfy the bacteria and since it is steamed, has no weed seeds. 99% of us cannot buy it as we are not near mint farm. Next best (only my opinion) is alfalfa hay mulch. It works great and is used a lot in Australia on dahlias. Downside is that it has some weed seeds. It has nitrogen n in it too. The simplest solution if you want to be organic, would be to use chicken feather meal as a source of nitrogen . It is a "time release" fertilizer that is said to be active for 3-4 months and that is most of our growing season for dahlias. A club member had his soil tested and the nitrogen was just fine after 6 months from the chicken feather application.

cmcramer

I saw no change....but it was only a couple weeks before an early frost killled all my dahlias! Looking over my 5 beds.....the issue was found in only two beds and was not variety specific....so that makes me agree that it is a soil-based problem.