dying plant

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I have a small yard with three fruiting guavas, some flowers, herbs and two 2-month old santol (also a fruit-bearing tree). I've noticed that one of the guava slowly whither and finally died while the other guava just a foot away from it remains healthy. Then the santol which is about one meter away from it also starts to whither and die. Then I've noticed too that the creeping herb five meters away starts to decay. I really wonder what's happening considering tha the other plants are healthy and without any signs of disease. What's wrong?