An unknown disease(s) emerged this spring (2000) in an onion field in southwestern Texas. Infected onion plants exhibited two symptom types, one with shoot proliferation, moderate stunting of plants, and light yellowish discoloration on leaves (A) and the other with only severe stunting of the plants (B). The bulbs of the infected plants collected from both symptom types were smaller than normal. When the aerial shoots were trimmed, the infected (but not asymptomatic or the severely stunted) bulbs produced multiple slender sprouts after storage in room temperature for about a month. These symptoms are characteristic of yellows diseases caused by phytoplasmas. Ten symptomatic (six with symptom type A and four with symptom type B) and ten symptomless onion plants were collected in early May from an onion field about 1 to 2 weeks prior to blooming. Total nucleic acid was extracted from 0.5 g of shoot tissues from each sample. Nested polymerase chain reaction (PCR) using universal primer pairs (P1/P7 followed by R16F2n/R16R2) previously designed based on 16S and 23S rRNA gene sequence (1,2) was employed for the detection of phytoplasma(s) present in the samples. Specific PCR products (all were about 1.2 kb) were heavily amplified from five samples with symptom type A and one with symptom type B. Three of the symptomatic plants showing symptom type B and five of the symptomless samples were scored as weak positives. Restriction fragment length polymorphism (RFLP) analyses of the PCR products obtained from all five symptomatic samples with symptom type A using restriction enzymes including MseI, HhaI, and HpaII revealed that the associated phytoplasmas detected belonged to aster yellows phytoplasma group (16SrI), subgroup A. RFLP analyses of PCR product from the sample with symptom type B indicated that the associated phytoplasma belonged to clover proliferation group (16SrVI), subgroup A (1). Since symptom type A resembles onion yellows reported elsewhere, we propose to adopt “onion yellows” to refer to the new onion disease occurring in Texas. However, correlation between a member of clover proliferation phytoplasma group and onion plants showing severe stunting could not be firmly established. A phytoplasma belonging to 16SrI-B is associated with onion yellows disease reported in Japan (1). This is the first report that onion yellows occurs in the United States.
References: (1) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (2) B. Schneider et al. 1995. Molecular and Diagnostic Procedures in Mycoplasmology, Vol. I, Academic Press, San Diego, CA.