November 26, 2004
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of spot type of barley net blotch caused by Pyrenophora teres f. sp. maculata in Uruguay
S. A. Pereyra and S. E. German, Instituto Nacional de Investigacion Agropecuaria (National Institute for Agricultural Research), INIA La Estanzuela, Ruta 50 km 11, 70000, Colonia, Uruguay. Plant Dis. 88:1162, 2004; published on-line as D-2004-0803-01N, 2004. Accepted for publication 8 Jun 2004.

In September 2003, leaves exhibiting spot-type lesions similar to those produced by _Cochliobolus sativus_ Drechs. ex Dastur were widely observed in 6 commercial barley crops of cvs. Nortena Dayman, Nortena Carumbe, and MUSA 936 in Soriano and Rio Negro provinces, the main barley production region in western Uruguay.

Spot lesions were tan to dark brown, circular to elliptical, and 3 to 10 mm in diameter. Larger lesions were surrounded by a chlorotic margin of varying width. Affected leaf pieces (10 to 15) from each field were placed
in a moist chamber for 2 days to promote sporulation. A fungus identified morphologically as _Pyrenophora teres_ (Died.) Drechs. (1) was consistently isolated from infected leaves. However, symptoms did not correspond to the net-type lesions of net blotch commonly produced by _P. teres_ f. sp. _teres_ in Uruguay.

3 monoconidial cultures were obtained by transferring single conidia to potato dextrose agar and then to 10 percent V8 juice agar and incubated at 20 to 22 C, with a 12-h photoperiod, for 10 days. Adding sterile water to each plate and gently rubbing the surface with a microscope slide prepared inoculum for pathogenicity tests. Conidia concentration was adjusted to about 10 000 conidia per ml.

68 barley genotypes from Uruguay, ICARDA/CIMMYT, and North Dakota were grown in the greenhouse for 2 weeks at 20 to 22 C with a 14-h photoperiod. For each monoconidial isolate, 3 seedlings of each genotype were inoculated at the 3-leaf stage 15 to 16 days after seeding with 0.4 ml of the inoculum suspension with an airbrush inoculator. A drop of Tween 20 was added per 40 ml of inoculum suspension. One set of each genotype was inoculated with sterile water as a control. Seedlings were placed in a dew chamber at 20 C and 100 percent relative humidity in the dark for 24 h and then returned to prior conditions.

The 1st lesions developed after 7 to 9 days. Leaves 2 and 3 of the plants were visually rated for disease (3) 13 days after inoculation. Control plants were disease free. The most susceptible reactions were observed on cvs. Nortena Dayman, MUSA 936, and line CLE 230 (Uruguay). Symptoms were similar in shape and size to those observed in the fields. The most resistant infection types were observed on several Uruguayan and North Dakota advanced lines.

The fungus was consistently re-isolated from inoculated plants. On the basis of morphology and symptoms produced, the pathogen was identified (2) as _P. teres_. f. sp. _maculata_ Smedeg.

To our knowledge, this is the 1st report of this fungus causing spot-like symptoms of net blotch in Uruguay.

References:
(1) M. B. Ellis. Dematiaceous hyphomycetes, CABI, Oxon, UK, 1971.
(2) V. Smedergaard-Petersen. Pages 124-144 in: R. Vet. Agr. Univ. Yearbook, Copenhagen, 1971.
(3) A. Tekauz. Can. J. Plant Pathol.7:181, 1985.

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[_Pyrenophora teres_ f. _teres_ [Ptft] (causing net blotch) and _Pyrenophora teres_ f. _maculata_ [Ptfm] (causing "spot form" of the disease) are important foliar pathogens of barley. Estimated losses worldwide are up to 20 percent. Isolates derived from single conidia were evaluated for their virulence phenotypes on 25 differential barley genotypes. In general, the Ptft isolates exhibited a broader spectrum and a higher level of virulence on the host differentials than the Ptfm isolates. 8 barley genotypes were resistant to all 19 pathotypes identified and
should be useful in breeding barley for resistance to both forms of _P. teres_.

Genetic variation was also examined by restriction fragment length polymorphism (RFLP) analysis. A 0.46-kb DNA fragment (ND218) generated by PCR from genomic DNA of a California isolate of Ptft was used as a probe. Every _P. teres_ isolate tested with ND218 exhibited a unique RFLP pattern. Cluster analysis, based on both the virulence phenotypes and RFLP patterns, indicates that _P. teres_ possesses a high degree of diversity at the
species and subspecies levels. The high degree of polymorphism revealed by ND218 will make this probe a useful tool for the DNA fingerprinting of _P. teres_ isolates. Sequence analysis of the 18s rDNA revealed 100 percent homology between the 2 forms. This intron distinguishes _P. teres_ from the barley leaf spot pathogen _Cochliobolus sativus_, the symptoms of which are often confused with those of Ptfm. The closely related _P.
tritici-repentis_, which causes similar spot lesions, also lacked this intron. DNA sequence analysis of the ITS1 and ITS2 spacer regions revealed only 1.6 percent divergence between the 2 forms.

Disease management involves the use of seed treated with fungicide, avoidance of direct sowing on stubble without ploughing, and starting fungicidal treatment according to local advice when symptoms occur on one of the last 3 barley leaves. When plants are severely attacked, applications of fungicide should commence at the one node stage to delay possible disease outbreak.

Links: <http://pubs.nrc-cnrc.gc.ca/tcjpp/k02-063.htmlhttp://pubs.nrc-cnrc.gc.ca/tcjpp/k02-063.html>
<http://www.inra.fr/Internet/Produits/HYP3/pathogene/6pyrter.htm#trt>
<http://www.apsnet.org/pd/summaries/dse01sum.asp>
<http://www.ucd.ie/agri/html/homepage/research_96_99/research_1998_99/ERM/ERM04.html>
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