December 3, 2005
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes, December 2005 [edited] <http://www.apsnet.org/pd/searchnotes/2005/PD-89-1360C.asp>

Fusarium wilt of tomato caused by Fusarium oxysporum f. sp. lycopersici Race 3 in Baja California Sur, Mexico
R. J. Holguin-Pena, Centro de Investigaciones Biologicas del Noroeste, Mar Bermejo 195, Col. Playa Palo de Santa Rita, La Paz, Baja California Sur 23090, Mexico. Plant Dis. 89:1360, 2005; published on-line as DOI: 10.1094/PD-89-1360C. Accepted for publication 31 Aug 2005.

Vascular wilting diseases have become one of the most serious diseases of tomato (_Lycopersicon esculentum_) throughout the Baja California Peninsula. Since the winter of 2004, a disease with symptoms characteristic of those caused by a Fusarium species has been observed in commercial fields near La Paz and Todos Santos in
the state of Baja California Sur (BCS). Symptoms include typical one-sided wilting and dark brown vascular discoloration.

Upper stem tissues and wilted seedlings were disinfested by immersion in a 1.0 percent aqueous solution of sodium hypochlorite for 2 min, rinsed in sterile water, and placed on Komada's medium (pH 6.8) at 22 plus/minus 3 deg C. After 72 h, hyphal growth was recovered and subcultured on carnation leaf agar and potato dextrose agar and incubated at 25 deg C in 12-h light/dark cycles.

Identification was based on colony morphology, conidial characteristics, and molecular techniques. White cottony mycelium, reddish coloration of the medium, ovoid 2-celled macroconidia, and large macroconidia, all characteristic of _F. oxysporum_, were observed (2). Polymerase chain reaction and restriction fragment length polymorphisms with restriction enzymes EcoRI, RsaI, and HaeIII were used to characterize 24 isolates (sampled during January 2005) from La Paz (Fol-LaP) as _formae speciales lycopersici_ and assigned to vegetative compatibility group 0030 (1).

Confirmation of pathogenicity and race determination for the Fol-LaP isolates were as described previously (3). Mexican isolates of races 2 and 3 (one each) were included as positive controls. Conidial suspensions of 700 000 CFU/ml were used to inoculate differential tomato cvs. Bonny Best (Millington Co., universally susceptible), Tequila F1 (Vilmorin, race 1 resistant), Rio Grande (Harris Moran, race 1 and 2 resistant), and Sebring (Rogers, race 1, 2, and 3 resistant).

Plants at the first true-leaf stage were inoculated by dipping their roots in the conidial suspension. Inoculated seedlings were transplanted into pots containing a sterile 5:1:1 mixture of sand/vermiculite/soil (v/v/v) and maintained in the greenhouse at 25 to 28 deg C under natural daylight. An equal number of plants of each
cultivar dipped in water were used as controls. The experimental design was a completely randomized type with 6 replications (pots) containing 4 seedlings per pot. The test was done twice.

The most susceptible plants inoculated by root-dipping developed typical symptoms of wilt, slight vein clearing on outer leaflets, stunting, dark brown vascular discoloration, and death. _F. oxysporum_ was recovered from all symptomatic plants, whereas noninoculated tomato seedlings showed no symptoms. According to differential infection and symptomatology observed on infected cultivars, 62.5 percent of the isolated strains were identified as race 2, 25 percent as race 3, and 12.5 percent as an undetermined race isolated from Sebring.

The presence of race 3 in BCS has important epidemiological implications, since it has been reported on tomato in Sinaloa (4). The potential spread of the pathogen on introduced transplants represents a risk to tomato crops on the peninsula.

To our knowledge, this is the 1st report of _F. oxysporum_ f. sp. _lycopersici_ race 3 in the state of BCS, Mexico.

References:
(1) G. Cai et al. Phytopathology 93:1014, 2003.
(2) P. E. Nelson et al. Fusarium species. Pennsylvania State University Press, University Park, 1983.
(3) B. A. Summerell et al. Plant Dis. 87:117, 2003.
(4) J. G. Valenzuela-Ureta et al. Plant Dis. 80:105, 1996

--
ProMED-mail
<promed@promedmail.org>

[Fusarium wilt, caused by 3 races of _Fusarium oxysporum_ f. sp._lycopersici_ [Fol] is one of the most important diseases of tomato. Races 1 and 2 are distributed worldwide, whereas race 3 has a more limited geographic distribution with no report thus far in Brazil. 7 Fol isolates were obtained from wilted tomato plants of race 1 and 2-resistant hybrids 'Carmen' and 'Alambra' in Venda Nova do Imigrante (State of Espirito Santo), Brazil. Results of virulence assays using a set of race differential cultivars showed that 'Ponderosa' were susceptible to all races, 'IPA-5' was resistant to race 1, 'Floradade' was resistant to races 1 and 2 and 'BHRS-2,3' was resistant to race 3. All isolates were highly virulent to 'Ponderosa', 'IPA-5' and 'Floradade' and were able to infect only a few plants of 'BHRS-2,3'. An additional virulence test was conducted including the same set of cultivars plus _Lycopersicon pennellii_ 'LA716'. Identical results were obtained with _L. pennellii_ displaying an extreme (immune-like) resistant response.

These results indicated that all 7 isolates could be classified as Fol race 3. This new Fusarium wilt might became an economically important disease, since race 3-resistant cultivars adapted to Brazil are not yet available.

This is the 1st formal report of race 3 in Brazil, extending the geographical range of this pathogen. Fol can be introduced into new growing areas via contaminated seeds. The presence of race 3 in geographically isolated tomato-growing areas in Brazil could be the result of either pathogen introduction via contaminated seeds or the
occurrence of an autochthonous (indigenous) race 3 isolate. In fact, recent results indicate that new race 3 isolates could have originated from genetic changes in the local populations of native Fol isolates. Additional studies employing molecular fingerprinting systems and/or vegetative compatibility groups with a world-wide collection of Fol isolates could provide some indication of whether this pathogen is endemic to Venda Nova do Imigrante or if it was introduced and, if so, from which geographical area.

This new Fusarium wilt might became an economically important disease since race 3-resistant cultivars adapted to Brazilian conditions are not yet available. In addition, screening trials searching for new sources of resistance seems to be necessary, since the genetic plasticity associated with selective pressures due to the use of race
3 resistant cultivars might cause the establishment of new pathogenic races of this fungus. These screening trials are already under way in Embrapa Hortalicas, and new resistance sources have been found.

Management strategies for Fol include disinfestation of the soil and planting material with fungicidal chemicals; crop rotation for several years, but Fol is long-lived; planting non-hosts of Fol; and using resistant cultivars. Limit the spread of infested soil by cleaning farm equipment. Avoid root knot nematode infestations, because nematode feeding can overcome plant resistance to Fusarium wilt.

Links:
<http://www.scielo.br/pdf/fb/v30n4/a17v30n4.pdf>
<http://web.aces.uiuc.edu/vista/pdf_pubs/929.PDF>
<http://www.cdl.umn.edu/pubs/pdfs/HCK/race3.pdf>
<http://www.cdl.umn.edu/pubs/pdfs/HCK/race3.pdf>
- Mod.DH]

[see also in the archive:
2001
Fusarium wilt, tomato - USA (Tennessee) 2000 20010721.1416]