Sentence:  The extremely low number of control plants infected by <WSD>stubborn</WSD> disease suggests that natural spread of stubborn in the vicinity of the experiments is much slower than that reported for psylla-borne greening in South Africa (Schwarz, 1964). 
Meaning of the word marked with <WSD> tags:  A disease of citrus trees characterized by stunted growth and misshapen fruit, caused by (ver=190511).