Spiroplasma citri (Saglio) causes citrus stubborn disease in most commercial citrus varieties resulting in reduced yield and fruit quality. This dissertation sought to improve detection of S. citri, monitor disease progress in the field, and determine yield losses of citrus with stubborn. We assayed Valencia orange (Citrus sinensis [L.] Osbeck) and grapefruit (Citrus paradisi Macfadyen) from the Coachella Valley and Navel orange (C. sinensis) from the San Joaquin Valley of California.; To detect differences in disease expression of the trees tested, we developed a severity rating scale based on visual symptoms in the canopy of individual trees: 0 = unhealthy, 1 = 1--10%, 2 = 11--25%, 3 = 26--50%, and 4 = >50% of a tree showing symptoms. We tested the efficacy of using PCR for detection of natural occurring citrus stubborn infection.; Spiroplasma citri was detected year round from fruit of severity Level 4 rated trees, and from young leaves and twigs from Level 4 trees for 9 months. PCR was not useful for detecting S. citri in fruit and twigs from trees showing mild symptoms, while detection was sporadic using young leaves from those trees.; We assessed the effect of citrus stubborn on three yield parameters: total fruit weight per tree, number of fruit per tree, and size of fruit. The total fruit weight from Navel and Valencia orange trees with Level 4 symptom severity was significantly lower than that obtained from trees with severity Levels 0, 1, and 2. The total fruit weight of grapefruit trees was not statistically different among all severity levels. Fruit number was statistically similar in all symptom severity levels in the three citrus types tested. The average weight per fruit, as well as the fruit diameter, decreased with increasing disease severity.; We monitored citrus stubborn disease progress in 12 groves over a three year period. Disease severity in grapefruit changed dramatically, from Level 0 to 3 in two groves. In contrast, Valencia orange had the lowest increase in disease severity. Navel orange demonstrated an intermediate disease severity reaction. Navel and Valencia oranges showed a marked stubborn symptom remission for about four months in the second year of evaluation.
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