Sentence:  ... [I]nfected tulips are weakened by the viruses that cause the very patterns and swirls that fascinated horticulturists and investors in the first place. Such bulbs tend to <WSD>dwindle</WSD> away instead of fattening up and producing offsets. 
Meaning of the word marked with <WSD> tags:  To fall away in quality; degenerate, sink.