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Disease Resistance

Fusarium Head Blight

Since 1993, Fusarium head blight (FHB) has cost over an estimated $3 billion in direct and indirect losses to North Dakota.  Durum production in North Dakota has declined because of FHB.  In 2001, North Dakota produced 0.9 million metric tons of durum wheat, 52% less than in the year 2000.  The decline in harvested hectares and durum production in North Dakota has adversely affected its economy and has directly impacted the U.S. pasta industry and the international export market.  Fusarium head blight reduces yield and the end-use quality of durum wheat. The fungus can produce mycotoxins, particularly trichothecene deoxynivalenol (DON vomitoxin), that are hazardous to humans and other animals.

Sources of resistance to FHB are available in hexaploid wheat.  Sumai 3, a Chinese hexaploid wheat, is one of the most prominent sources examined for Type II resistance (spread of the disease in the infected heads).  Sumai 3 has been widely used in the world as a source of resistance to develop FHB-resistant wheat cultivars.  A source of resistance similar to Sumai 3 in durum wheat is not available.  We have screened lines from North Dakota, Canada, CIMMYT (International Maize and Wheat Improvement Center, Mexico D.F., Mexico), ICARDA (International Center for Agricultural Research in the Dry Areas, Aleppo, Syria), and all accessions from the world collection which represents germplasm from the Mediterranean region (Italy, Spain, and Greece) and North Africa for FHB resistance.  The accessions were obtained from the USDA National Small Grains Collection, and the project is funded by the U.S. Wheat and Barley Scab Initiative. Our breeding efforts resulted in t releasing the cultivar Divide in 2005 that has some level of resistance to FHB.

 

Tan Spot

tan spotTan spot caused by Pyrenophora tritici-repentis (Died.) Drech. anamorph Drechslera tritici-repentis (Died.) Shoem. is a major foliar disease affecting durum and bread wheat produced in North Dakota.  The adoption of conservation tillage practices and lack of adequate genetic resistance in modern wheat cultivars have increased incidence of tan spot. Economic losses due to tan spot epidemics can be expressed in lower grain yield, reduced kernel weight, and pink kernel discoloration.

Sources of resistance to tan spot that are being used in the breeding program.  The modified pedigree selection method is being used to develop durum wheat cultivars resistant to tan spot. Greenhouse and field tan spot evaluations begin at the F5 progenies and subsequent generations. In 1996, the breeding program released the durum wheat cultivar Ben that has a good level of resistance to tan spot.

 


Rusts

rustsIn 1950, 1953, and 1954, the durum wheat crop in North Dakota was severely affected by the race 15B of stem rust caused by the fungus Puccinia graminis pers.:Pers.f. sp. tritici Eriks. & E. Henn.  Breeding durum wheat cultivars that were resistant to race 15B became a major objective of the breeding program.  Before 1950, many of the durum wheat cultivars had the Sr9e allele that was transferred from the Vernal emmer.  The Sr9e allele provided a high level of resistance until race 15B became prevalent in 1950.

‘Khapli’ and ‘Vernal’ were identified as having resistant genes to race 15B and were widely used in breeding programs.  Khapli emmer was used as a source of resistance in the North Dakota breeding program to release durum cultivars Lakota, Langdon, Yuma, and Wells that were resistant to stem rust.  Khapli was reported to have at least four genes for resistance; three of these genes are probably Sr7a, Sr13, and Sr14.  Other sources of resistance such as St 464 and Wells have been used in the breeding program.  The current durum germplasm including the durum wheat cultivars in North Dakota are highly resistant to stem rust.  This is possible because through the years, the breeders have pyramided Sr genes into the germplasm.  The search for new sources of resistance to combat new virulent races of stem rust that may arise is a continuous process in the breeding program.  The North Dakota germplasm is screened against the pathotypes Pgt-QCC, -QTH, -RTQ, -RCR, -TML, and –HPH.  These pathotypes are isolates that were collected from North Dakota by Dr. James Miller, plant pathologist, USDA-ARS, and characterized using the international system of nomenclature for stem rust that was proposed by Roelfs and Martens (1988).

Leaf rust caused by the fungus Puccinia triticina Erick can attack durum, but does not cause serious yield losses as stem rust.  Most durum wheat cultivars in North Dakota have a low coefficient of infection and, therefore, are little damaged when infected with leaf rust.  D561, D6733, ‘Ramsy’, and ‘Leeds’ have been used as sources of slow leaf rusting in the breeding program.  All the current durum wheat cultivars developed in North Dakota are highly resistant to leaf rust.  The germplasm is continuously screened for leaf rust to combat new races.

Stripe rust caused by Pucccina striiformis West. is not a serious disease in North Dakota. Research on stripe rust is limited in North Dakota, compared to that on stem and leaf rusts.

 

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