Well I am at an all time version of "stumped" with an issue in my annual garden plantings. We live in North Central Georgia [30656], and have both Vinca flowers and (perrinial) Mexican Heather, growing in our front full sun garden beds. The soil is mostly a red clay mix, and the majority of the plants in the beds, are doing beautiful. However I have six plants (spots) in various areas of my front garden bed, that simply refuse to grow no matter what we have tried both in irrigation and fertilizer and how many times I have replaced the plant in that particular spot.
A bit of background this is the very first year in four years since moving to the area, that we have finally placed annuals in the ground. Last year, as we were starting our landscaping plan, we placed the Mexican Heather into the ground last spring. It grew fine, and this year, all but two plants came back this season without any issue. These two plants were most likely killed off because they had been mistakingly been planted with the weed guard barrier under the roots. They were replaced and are growing without any further issues.
We planted the Vinca in April. Almost 90 feet of beds. All of the plants stared out normal, but these six plants in completely random areas, seemed to have an issue right from the start. At first we thought that a night of lower than normal temps, may have caused the death of these plants, and they were replaced. However, Within a short period of time, those new plants also died. At the time, I was not paying too much attention to which plants were needing to be replaced. At this them I was simply thinking that a few plants were randomly just not making it, and they were replaced as they died. The issue really started ,to peak our interest when we started to notice that the plants that were dying, seemed very close to the area that we had replaced before. Because we have a very large lawn with many plants, it did not seems out of the ordinary at first. But since and I am disabled with a back issue and lupus, my husband does the planting for me AKA: point and plant, and now we had two sets of eyes on these plants, and really started to notice that it was indeed not random, but the same plants in these random areas of the beds were dying. And yes, we were now on a on aTHIRD replanting, and we without a doubt realized that it was actually these certain individual that plants, were having reoccurring 'death issues'.
First up, we looked at the irrigation. Upon closer look, it seemed that the pop up sprinkler heads, were [possibly] not totally reaching these particular plants effectively. And we adjusted the spray of the heads (and also put in ANOTHER vinca). We watched it for two weeks, and they seemed to be doing better. And then, they took a turn for the worse and just died off. We looked closer at the irrigation. While the ground looked like it was being hit by the water in those spots, it seemed like just wasn't hitting those exact areas in the same way, and was not enough to keep the new plants alive.
The pop up spray heads were replaced by direct drip hoses. The spacing of the new drip holes on the hoses, was every six inches. In the time that it took us to replace all of the pop up heads and lay new the new drip hoses, the now FOURTH planting of vinca in those certain spots were also now dying (yet again). We thought that somehow we just may have possibly not gotten in the new irrigation in time to save these newly planted spots. We also thought that perhaps the above normal heat of 90+ degree days, was just too traumatic for those new plants and we AGAIN put in a new plant into these six spots that are simply refusing to let any plant grow. I think that brings us to five or six replacement plantings now? Needless to say the head scratching involved here is causing bald spots!
The spots where the plants are dying, are totally random in the 90 ft beds. In three areas, the plants to the left and right of the victim plants, are growing beautifully. It is only in one particular area where there are three vinca plants in a row, that won't thrive.

Now , I am no stranger to Vinca flowers. I know that Vinca do not like 'wet feet', and are careful to not over water the beds. And while our summer has been incredibly hot and dry, Conditions such as this are normally considered to be absolutely perfect conditions for vinca to thrive. It is without a doubt the strangest most oddball gardening issue I have EVER run into! It is only in these particular six spots, and over approximately 90 feet of flower beds, where the rest of the flowers are growing great. I would even use the word lush to describe the other flowers in the exact same bed.
I'm not sure if possibly the very first plants that were put into the ground, had any disease issues and contaminated every future plant? I don't know if that is even possible! All I know is I am left totally scratching my head and stumped in a way that I have never been stumped before in all of my 35+ years of growing both flowers and veggies. I consider myself to be very well versed in knowing what will or will not grow in certain areas. When I don't know, I always do my homework, especially when it comes to perrinials and putting in any 'permanent plants' into any area. Because I have always had a home that had full sun flower beds, I have always chosen Vinca over imaptiens for the 'oomph factor' of color. So I consider them to be a flower variety that I am comfortable planting. I have never had any issue simply because of how EASYGOING they are to get to thrive every summer! Below are pictures of two the spots where the issue is taking place. If needed, I can supply more photos to show the health of the entire bed(s) as a whole. It is literally these six random spots in the ground, where I simply cannot get the vinca to grow to match the rest of the beds.

Vinca (periwinkle) are susceptible to nematodes and fungus. I have heard if you have a problem with them do not replant new ones in the same place. Because they are a tender perennial, I have mine from last year, you should periodically move them and plant something else where they were. I understand these were new for you. I would think fungus is your problem. Whether the fungus was in the plants to begin with or in your soil it would stand to reason the fungus is now in the soil. Sometimes baking soda washes or hydrogen peroxide washes can help the soil or you could try a fungicide. Sorry I do not have any recommendations on fungicides.

If it is disease, and you know someone willing to move many wheelbarrows of soil, you might have an opportunity to keep it from spreading to the rest of your bed. Excavate those spots, replace with known-clean soil, and use the questionable soil to improve some bed that will never be used to grow Vincas. Like shrubbery or vegetables.

Then, plant something as different as possible from Vincas in and around those spots for several years, to reduce the number of spores in the soil.

But it surprises me that any soil disease did not spread rapidly. Do you follow "no-till" practices?

I'll mention another possibility only because I am always obsessed with drainage. When you remove the plants, do you inspect the roots and the soil they werein? Big fat healthy-looking roots? Just a few skinny threads? "Open" soil with air spaces, or glueey clay?

If the red clay soil was not well mixed with amendments, there might be "clayey spots" that hold water, especially if the slow-draining zone was shaped like a cup or bowl.

But you could rule that out with a little spade work or forking around those spots, when you replace the plants next.

If you don't till and mix your soil, maybe there are clumps with bad drainage right in those spots. Or big rocks near the surface.

One thing you could do to rule out some possibilities would be to put a slightly raised bed in those spots, so the plants have some root zone above grade, assuring at least some aerated root zone near the surface. Maybe just mound the soil up before re-panting, or use some kind of edging to support 3-6" of raised soil.

Here is another possibility, as bad as soil disease.

Could someone before you have dumped oil or herbicides or other unhealthy chemicals in just a few spots?
If you have a crazy neighbor on the warpath, could they have sprayed herbicide in just a few spots?

Could you have gotten a batch of soil amendments with (for example) very saline clumps, or too-strong fertilizer, or herbicide residue? Manure with salt and/or herbicides?

Ah -- I feel your pain, @ajerseyjee .

I managed to start vinca from seed this year (which has not been successful for me in the past), only to have quite a few of my plants succumb to spider mites.

My plants are in flower boxes on the south side of our deck, so I hope they will like hot, sunny conditions. (and that they recover enough to put on a decent display of flowers this year!)

Hi,

I just saw this post as I was heading to bed. I'm too tired to read it closely but I skimmed through the post and looked at the photos. I'm copying and pasting a post that I recently made elsewhere and posting it here because I think it is the same issue. I'll read your post again tomorrow when my eyes will stay open.

It sounds and looks like you could have a Phytophthora infection. Some sources say it is a fungus while others call it a water-mold. No matter what the name, it is a destructive disease that is usually fatal to "Vinca" aka Catharanthus roseus. It spreads by spores and flourishes in wet conditions. Planting Catharanthus roseus in wet soil, over-irrigating, or an excess of rain sets up the perfect conditions for the disease to run rampant. Also, planting Catharanthus roseus too early in the year while the soil is still cool will also give the disease an edge. Catharanthus roseus likes drier, warm soil. I've also seen plants for sale many times that were clearly infected so the spores could have come on the first plants that you bought. Check out this link:
http://www.plantanswers.com/vi...

If this is the problem, the spores are now in the soil so don't bother trying to replant in the same spot as the infected plants will become infected, too. The infection can continue to rapidly spread from one plant to the next in a domino effect as the spores spread. You'll need to pull up infected plants and dispose of them in the trash. Do not compost them. If you have your heart set on more Vinca, there are a few resistant varieties, such as 'Cora'. I've grown 'Cora Apricot' before and it performed really well and has a lovely color. There are several nice color choices in the line.