Sometimes Vinca can suffer from bacterial diseases such as botrytis. Those plants stress easily when transplanted into a garden and the yellowing leaves are a common symptom. However, if the weather is warm and dry, the plants can often grow and hardly show any symptoms. Rainy and cool weather produces more symptoms.
How long ago did you plant these Vinca's, I have often had the same problems here in UK, all in the same area, first check the soil to see if there is any difference in PH, as there can be from one yard to the next, also same with water, is there rubble under the soil of the poor looking one, this could be a prob, has there been over feed / under fed, there are many things that could be the reason even just a plant that really wont grow well no matter what you do, not that I'm saying that is the case here, the best thing to do is make a list of general troubles that COULD be with the plant and tick them off as you examine your lists, correcting if you can as you go along,
Hope the problem becomes evident soon as believe me, I know what frustration cane take over while your seeking answers.
Hopefully someone who has had your problem as seen in picture can identify what you need right away.
Best of luck. WeeNel.
this is the first of 3 plants transplanted in the same area a couple of months ago. i transplanted the 2nd and third plant at the same time, maybe 3 weeks after the first plant. only the first plant developed symptoms. it's the monsoon season here so it's been raining a lot though, i'll try to look for rubble under this particular plant maybe this causes poor water drainage. I just hope it's not a disease as this one has the best flower color among the three. Thanks Cville_Gardener & WeeNel for the help. =)
I've had the yellowing come and go like that with wet weather. Could you dig that one and put it in a pot until monsoon season is over? They will thrive in containers. Or moving it to a spot with better drainage might do the trick. If it is diseased, getting it away from the other plants would be advisable anyway.
I have problems with Vinca I sowed from seeds I colectected from last year.They took a long time to sprout in doors I put them under grow light and I got them climatized when the temp was good but I ended up with one plant that I put in my container.