Cucurbit Genetics Cooperative Report 17:72-73 (article 19) 1994
A.I. L. Sese, M.L. Gomez-Guillamon, J.R. Diaz-Ruiz
Estacion Experimental “La Mayora” CSIC, 29750 Algarrobo-Costa, Malaga SPAIN, Centro de Investisgaciones Biologicas CSIC, Velazquez 144, 28006 Madrid, SPAIN
A previously unknown melon yellowing disease associated with the presence of Bemisia tabaci (5) has been found in melon, Cucumis melo L., cultivated under plastic greenhouses in southeastern Spain. In this area, the vector of melon yellowing disease virus in melon was Trialeurodes vaporariorum (9) but since 1989 this has been progressively displaced by Bemisia tabaci. In some crops, the two vectors were found co-existing.
Experiments were started to study the causal agent-vector relationship and the nature of the pathogen to help the search for possible sources of resistance to this new melon yellowing disease.
In advanced stages, it is difficult to differentiate the symptoms from those of yellowing disease transmitted by Trialeurodes vaporariorum (5) but, in the initial stages, chlorotic leaf spots are usually more diffuse. early symptoms appear around 20 to 22 days after inoculation and are well established at 30 days.
The causal agent was not transmitted mechanically by the two extraction techniques employed in this work: using either phosphate buffer 0.1M pH 7, or the same buffer plus DIECA, a compound usually needed for difficult-to-transmit viruses.
The efficiency of B. tabaci as a vector of the pathogen was determined with a series of seven plant groups inoculated with 1, 5, 10, 20, 30, 40, and 60., B. tabaci individuals that had previously been allowed to feed for 48 h on plants showing symptoms. In three replicates of the experiment we observed that, although one individual of B. tabaci transmitted the disease, 100% transmission required 60 individuals.
The persistence of the infective particle inside the vector was determined by inoculating batches of plants each day with 50 individuals of B. tabaci that had previously been allowed to feed for 48 h on plants showing clear symptoms; three replicates were used for this experiment. The results revealed that transmission occurred until the seventh day after inoculation and this indicated that the causal agent-vector relationship was of the semi-persistent, non-circulative type described by Duffus (4).
To study the minimum period needed to acquire the pathogen and transmit it to healthy plants, six groups of 50 B. tabaci individuals were allowed to feed on plants showing symptoms for one of the following periods: 2, 6, 18, 24, 48, and 54 h. Afterwards, the groups were transferred to healthy plants and allowed to feed for 72 h. Initial results showed that two hours of feeding on plants showing symptoms were sufficient for the individuals of B. tabaci to acquire the pathogen and to transmit it to healthy plants in 50% of cases. Twenty-four hour feeding times gave slightly less than 100% transmission.
Electron microscopy of differentially-centrifuged concentrates of sap extracted from plants that showed symptoms revealed rod-shaped and flexuous virus particles around 790 nm long that were not present in sap from healthy control plants.
The symptoms of this disease and the morphology of the causal virus appear very similar to those attributed to YSD (7) and LIYV (3) and transmitted by Bemisia tabaci, and to CuYV (10), to MYV (8) to BPOYV (2), and to another yellowing disease of melon (6), transmitted by Trialeurodes vaporariorum, although in our case, the virus particles are appreciably shorter.
To seed sources of resistance, 45 accessions of C. melo and related species from widely separated areas in Spain and from other parts of the world were evaluated under natural infection conditions. The transmission tests showed that the yellowing was transmitted by Bemisia tabaci, but not by Trialeurodes vaporariorum. The C. dipsaceus, C, metuliferus, and C. longipes species and six accessions of C. melo showed no symptoms.
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