Detection of the carrot bacterial leaf blight pathogen, Xanthomonas campestris pv. carotae, was achieved using polymerase chain reaction (PCR) along with primer pairs developed from sequences of cloned random amplified polymorphic DNA (RAPD) fragments. Primer pairs 3S and 9B directed the amplification of ∼350-bp and ∼900-bp (or ∼2 kb) DNA fragments, respectively, from genomic DNA of all known X. campestris pv. carotae strains tested, but not from that of 13 other X. campestris pathovars or other bacterial species, including yellow non-xanthomonad bacteria isolated from carrot tissues and seeds. In tests conducted with an extensive collection of X. campestris pv. carotae-like strains isolated from different substrates from California, Idaho, Oregon, Washington, and Canada, the 3S primer pair directed the amplification of the ∼350-bp target fragment from all strains. These results indicated that the 3S primer pair is highly specific for X. campestris pv. carotae detection. Using the 3S primer pair, PCR assays were developed for detection of X. campestris pv. carotae from colonies on agar media, carrot leaf and stem tissues, and seeds. These tests could be performed in a single day. The PCR-based seed assay detected X. campestris pv. carotae from lots with contamination rates ranging from 2 × 102 to 2.3 × 108 CFU per gram of seed. This assay gave results similar to a seed-wash dilution plating assay and proved more sensitive than an enzyme-linked immunosorbent assay (ELISA)-based assay.
The relationship between levels of carrot (Daucus carota subsp. sativus) seed contamination with Xanthomonas campestris pv. carotae and (i) establishment of populations of X. campestris pv. carotae on carrot leaves and (ii) the incidence and severity of carrot bacterial blight was determined in field plots in Davis, California, in 1995 and 1996. Levels of seed contamination ranged from 0 to 1.5 × 105 CFU/g in 1995 and from 0 to 1.5 × 107 CFU/g in 1996. Seed contamination levels were positively correlated with X. campestris pv. carotae populations detected on leaves and with the incidence and severity of carrot bacterial blight. The size of X. campestris pv. carotae populations on leaves was also directly related to disease incidence. In 1996, yields were significantly reduced in plots established with seed lots having the highest levels of X. campestris pv. carotae contamination. Under the conditions of this study (i.e., a location having low rainfall and relative humidity), the threshold of seed contamination for the establishment of X. campestris pv. carotae populations on leaves and for the development of carrot bacterial blight was unexpectedly high: 104 to 105 CFU/g of seed.
Leaves of honeydew melon plants infected and dark green mosaic symptoms. with watermelon mosaic virus showing lightHoneydew melon plants infected with cucumber mosaic virus showing yellow and green mosaic symptoms along with slight puckering of leaves. Infected plants usually appear stunted.
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