Citrus Bacterial Disease Research Articles

Latency and incubation of ‘Candidatus Liberibacter asiaticus’ in citrus after vector inoculation

Huanglongbing (HLB) is a bacterial disease of citrus, transmitted by the Asian citrus psyllid Diaphorina citri. In Brazil, the disease is managed by insecticide spray targeting vector control and removal of trees when they first show HLB symptoms that aims to reduce ‘Candidatus Liberibacter asiaticus’ (CLas) inoculum within the orchard. Albeit symptoms of HLB in field trees appear only months or years after the initial infection, the shortest incubation period (time between pathogen infection and symptom expression) reported for CLas in citrus was about 4 months after inoculation in studies in which this pathogen was graft-inoculated. In this work, an incubation period of 80 days (2.5 months) was recorded for citrus seedlings and 5 months for citrus nursery plants after inoculation with the psyllid vector itself. Psyllids were capable of acquire CLas from the infected seedlings and nursery plants 2.5 months after inoculation. These results show that the latency of CLas in planta is shorter than the incubation. Assessment of the presence of liberibacter in psyllids could be used as a proxy for the presence of HLB bacterium in presymptomatic stage of the plants.

Influence of rootstock variety on Huanglongbing disease development in field-grown sweet orange (Citrus sinensis [L.] Osbeck) trees

Huanglongbing (HLB), a bacterial disease of citrus, is causing substantial economic losses to the citrus industry worldwide. Sweet oranges are highly susceptible to the disease, and account for nearly 90% of all varieties grown in Florida. Rootstock is an important component of commercial citrus production, and tolerance to HLB has been reported for some rootstock varieties. Our objective was to investigate if rootstock selection has an effect on HLB disease development under natural conditions in the field. Four field trials with sweet orange scion on 15 different rootstocks were evaluated for incidence of Candidatus Liberibacter asiaticus (Las), the suspected causal agent of HLB, as well as incidence of foliar disease symptoms, canopy damage, and stem growth during the first years after Las began to spread into the trials. Trials ranged from two to nine years in age and included hybrids of trifoliate orange along with other rootstocks standard for citrus production in Florida. Fruit yield and soluble solids content were analyzed for the oldest trial. Our study showed that rootstock did not affect disease incidence and that trees on all rootstocks were considerably damaged by HLB. However, tolerance to HLB was higher in trees grafted on some rootstock selections. In the youngest trial, stem diameters on Volkamer lemon increased 53% while trees on US-852, Benton citrange and Swingle citrumelo grew the least at 21–26% from 2008 to 2010. Depending on trial and time of observation, foliar HLB symptoms on US-897 were less than on many other rootstocks, with ranks of 3.3–4.0 compared with 4.5–4.9 observed for US-812, US-852, Carrizo citrange and Kinkoji. Canopy damage ranks were 1.4–2.6 for trees on US-802 compared with 3.9–4.1 for trees on US-812, U-852, Sour orange, and Swingle in the oldest trial. In the youngest trials, trees on Volkamer showed least canopy damage while trees on US-852 and Benton were more affected. Highest fruit yields of 30–64kg/tree were obtained from trees on US-802 and Carrizo. In 2010, highest juice soluble solids content of 2.8–2.9kg was observed for trees on US-897 and US-812.

Use of carnauba based carrier for copper sprays reduces infection by <i>Xanthomonas citri</i> subsp. <i>citri</i> and <i>Diaporthe citri</i> in Florida commercial grapefruit groves

Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is a bacterial disease of citrus and results in peel blemishes rendering fresh fruit unsalable. Xcc is most active in warm, wet Florida summers where tissues are infected during periods of active growth. Melanose, caused by Diaporthe citri, is common in citrus producing countries, but, like canker, is only important for fresh market fruit. To control canker and melanose, Florida growers spray trees with copper formulations (Cu), but these sprays are removed by strong rains and intense radiation of Florida summers. A study was undertaken in FL commercial grapefruit groves in 2009 and 2010 to assess the efficiency of a spray combining copper with a specially formulated, hydrating wax (WashGard?) (WG). Using a 21-day spray schedule for the season, fruit were sprayed with WG + Cu, Cu and Control (no spray). Fruit from trees sprayed with WG + Cu had approximately 10 and17% more canker free fruit in 2009 and 2010 respectively compared to trees sprayed with copper alone. Compared to control trees the canker free fruit incidence was increased by ≈10% in 2009 and 57% in 2010. For melanose there was 40% more disease free fruit (treated) over fruit from trees with no treatment in 2009 and approximately 20% more in 2010. Controlling infection with this spray significantly reduces citrus canker and melanose, increasing the percentage of marketable fruit.

Bacterial Brown Leaf Spot of Citrus, a New Disease Caused by Burkholderia andropogonis.

A new bacterial disease of citrus was recently identified in Florida and is here named bacterial brown leaf spot (BBLS) of citrus. BBLS-infected citrus leaves from the field displayed circular, brownish, flat lesions with slightly raised and water-soaked margins surrounded by a chlorotic halo. Based on Biolog carbon source metabolic "fingerprinting", fatty acid analysis, and sequence analysis of partial 16S rDNA, gyrB, and rpoD genes, the causal agent of the disease was identified as Burkholderia andropogonis. Pathogenicity of these B. andropogonis isolates taken from multiple citrus leaves with BBLS was tested by various inoculation methods on three species of citrus as well as on carnation, corn, and sorghum. All isolates infected carnation, corn, and sorghum with varying degrees of pathogenicity. Variation among citrus isolates in pathogenicity was also observed in high titer (108 CFU/ml) inoculations of citrus leaves, ranging from a hypersensitive-like response to canker-like lesions. When the inoculum concentration was low (106 CFU/ml), only necrotic spots or small lesions slowly developed with all strains. Growth of B. andropogonis in citrus was relatively slow, tissue wounding appeared necessary for symptom appearance with many isolates, and field samples were recovered only after severe storms, indicating that this wide-host-range bacterium is a weak, opportunistic pathogen of citrus.

A Bacterial Disease of Citrus

DR. ETHEL DOIDGE, mycologist to the Department of Agriculture of the Union of South Africa, who is becoming well known for her researches into the bacterial diseases of plants, is to be congratulated on the excellent piece of work which is described in detail in an article on “A Bacterial Spot of Citrus.”1 At a time when research in phytopathology is largely at a standstill, it is refreshing to read of such ably conducted scientific investigations in our Colonies as these are.