Document Type

Dissertation - Open Access

Award Date


Degree Name

Doctor of Philosophy (PhD)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Shaukat Ali


isolates, Ptr toxins race, stem rust, tan spot, wheat, rye


Tan spot, caused by the ascomycete fungus, Pyrenophora tritici-repentis (Ptr), is a major foliar fungal disease of wheat worldwide. To date, Ptr isolates have been grouped into eight races based on the production of three host selective effectors (toxins), Ptr ToxA, ToxB, and ToxC, which are associated with necrosis and chlorosis symptoms on wheat differential lines. In order to see the correlation between stem rust resistance and tan spot susceptibility in 1970’s, to know the pathogen virulence profile, and explore the sources of tan spot resistance, we recovered isolates from wheat and rye and evaluated wheat and rye lines with different Ptr isolates. To determine the reaction of 247 prestem rust-epidemic wheat genotypes (n=156 bread and n=91 durum) against stem rust (TTKSK) and tan spot Ptr race1 (SD-13-101) and host-specific toxin Ptr ToxA, we observed 69% of the both bread and durum wheat cultivars were susceptible to both tan spot and stem rust. Thirty-nine percent of both bread and durum wheat genotypes gave differential responses (Susceptible/insensitive and Resistance/Sensitive) to race1 and Ptr ToxA respectively. Through genotyping 903 Ptr isolates we found majority of the islates with ToxA gene. A few isolates had the ToxB gene. Of the 903 isolates recovered, 273 were phenotyped for their race characterization using the wheat differential set. About 77 % of the isolates were designated as race 1, whereas 41% were identified as race 4. Most of the race 4 isolates were recovered from rye (77%). Only 4.3% isolates (one from SD and eleven from Baltic region) were grouped as race 2 and two isolates were classified as race 5 from SD. Twenty-four isolates identified which induced necrosis and chlorosis in Glenlea (ToxA sensitive) and 6B365 respectively or necrosis only in Glenlea but did not harbor the ToxA gene. Rye lines (39%) showed more susceptibility to race 5 when compared to race 1(21%) and were insensitive to both Ptr ToxA and Ptr ToxB. Based on our study, no correlation of stem rust resistance and increase in tan spot occurrence was observed. Similarly, Ptr ToxA is not the only factor responsible for the increase of tan spot disease in 1970’s and beyond. Our results show races 1 and 4 are largely prevalent on rye in the region with high frequency of race 4 as those isolates are avrulent on wheat suggesting its minimal role in the disease epidemiology. Rye could serve as a good source of resistance if needed. In terms of diversity of Ptr isolates, results of our study indicate that a wide range of virulence variation profile present in wheat in South Dakota, Baltic region and Romania, and some of the Ptr isolates from SD, Lithuania and Romania which did not harbor ToxA and did not fit into the currently prevalent 8 races. Ptr race 5, observed for the first time from SD.

Library of Congress Subject Headings

Wheat -- Disease and pest resistance.
Fungal diseases of plants.
Leaf spots.
Puccinia graminis.
Wheat rusts.


Includes bibliographical references



Number of Pages



South Dakota State University



Rights Statement

In Copyright