|Subbarao, Krishna - UC, DAVIS|
|Vallad, Gary - UC, DAVIS|
Submitted to: Eucarpia Conference on Lettuce and Leafy Vegetables
Publication Type: Abstract Only
Publication Acceptance Date: September 20, 2006
Publication Date: April 20, 2007
Citation: Subbarao, K.V., Vallad, G.E., Hayes, R.J. 2007. Verticillium wilts and management with special emphasis on lettuce. In Eucarpia Leafy Vegetables 2007 Conference Abstracts. University of Warwick, UK, April 2007. pp. 15. Technical Abstract: Verticillium wilt causes billions of dollars in annual losses worldwide. The genus Verticillium contains several phytopathogenic species, the most important being V. dahliae, V. albo-atrum, and V. longisporum. The soilborne habitat of these species, and their capacity to infect a variety of crops, make them chronic economic problems in crop production. Verticillium dahliae, the primary causal agent of Verticillium wilts, causes diseases on a wide variety of agricultural hosts in temperate and subtropical regions. The pathogen has great genetic plasticity and limited host specificity. Thus, wilt problems on one crop are likely to extend to other crops that follow. The list of the hosts infected by V. dahliae is also continually expanding, as new hosts succumb to the pathogen. A case in point is lettuce, a major crop in California, comprising 65% of the total acreage in the United States and an estimated value in 2004 of $1.93 billion. Lettuce was not even considered a host of V. dahliae until 1995, when the disease appeared suddenly in coastal California. The disease has since spread to many coastal production areas. In heavily infested fields, entire crops have been lost to the disease. Verticillium dahliae produces structures known as microsclerotia that can survive relatively long periods in soil, where they remain dormant until stimulated by plant roots. Microsclerotia germinate in response to nutrients released by a growing root, and grow towards the root. The fungus may then succeed in establishing an infection that will grow into the cortex of the root. Infections limited to the cortex cause little visible damage to the root, and many such infections may be sustained with little impact on yield. Verticillium wilt develops only after one or more cortical infections extend into the xylem and become systemic. Once in the xylem, the pathogen progresses acropetally through lateral roots, eventually colonizing the taproot and crown, whereupon the development of foliar symptoms occurs. The pathogen produces microsclerotia in infected lettuce plants that are returned to the soil with crop residue. The demonstrated capability of V. dahliae to cross-infect a variety of crops, reside in natural flora and invasive weeds, and become seedborne in various plants makes Verticillium wilt a chronic problem for many crops. Isolates of the pathogen infecting lettuce are also pathogenic on other crops grown in coastal California. Hence, traditional crop rotations used for managing other diseases are likely to fail for Verticillium wilt. Methyl bromide in combination with chloropicrin, has been routinely used for soil fumigation because of its low vapor pressure and broad-spectrum biocidal activity. However, for most crops, including lettuce, routine fumigation of infested fields is cost-prohibitive and does not assure long-term soil remediation. Therefore, the development of lettuce cultivars resistant to Verticillium wilt is the most optimal long-term solution. The testing of lettuce germplasm in field and greenhouse experiments has identified resistance to V. dahliae in a few lines that are currently being used in the breeding program. However, an unexpected amount of diversity exists among isolates of V. dahliae from different hosts, and further studies revealed that this variation even extends to those isolates collected from lettuce. To test the breadth of the resistance identified in lettuce, a pair of susceptible ('Salinas' and 'Sniper') and resistant ('La Brillante' and 'Little Gem') lettuce cultivars, were used as differentials and individually inoculated with 15 isolates of V. dahliae from lettuce in replicated greenhouse experiments. Both 'Salinas' and 'Sniper' were susceptible to all 15 isolates from lettuce, with variation in virulence exhibited among isolates; whereas, 'La Brillante' and 'Little Gem' were only susceptible to 2 of the 15. The two resistance-breaking isolates also formed a phylogenetic subgroup distinct from the other 13 isolates based on sequencing of the intergenic spacer (IGS) region of the nuclear ribosomal RNA gene. Accordingly, the two isolates of V. dahliae virulent on all tested cultivars were designated as race 2, while the remaining 13 that were virulent only on the susceptible 'Salinas' and 'Sniper' were designated as race 1. This is the first description of distinct virulence phenotypes in V. dahliae, since a similar race structure was described in tomato in the 1960s. The existence of resistance-breaking race 2 isolates prompted further screening of Lactuca germplasm for resistance to race 2. As current efforts to introgress race 1 resistance into cultivars progresses, it will be important to develop a strategy that deploys these resistant cultivars to fields dominated by race 1 isolates. While such a targeted approach is likely to extend the durability of resistance to race 1, it will also require the further development of markers to distinguish race 1 and 2 populations in the field.