**Methodical Considerations and Resistance Evaluation against** *Fusarium graminearum* **and** *F. culmorum* **Head Blight in Wheat. Part 3. Susceptibility Window and Resistance Expression**

### **Andrea György, Beata Tóth, Monika Varga and Akos Mesterhazy \***

Cereal Research Non-Profit Ltd., 6701 Szeged, Hungary; gyorgyandrea88@gmail.com (A.G.); beata.toth@gabonakutato.hu (B.T.); varga.j.monika@gmail.com (M.V.) **\***

 Correspondence: akos.mesterhazy@gabonakutato.hu

Received: 17 March 2020; Accepted: 22 April 2020; Published: 25 April 2020

**Abstract:** Flowering is the most favorable host stage for *Fusarium* infection in wheat, which is called the susceptibility window (SW). It is not known how long it takes, how it changes in different resistance classes, nor how stable is the plant reaction in the SW. We have no information, how the traits disease index (DI), *Fusarium*-damaged kernel rate (FDK), and deoxynivalenol (DON) respond within the 16 days period. Seven winter wheat genotypes differing in resistance were tested (2013–2014). Four *Fusarium* isolates were used for inoculation at mid-anthesis, and 4, 8, 11, 13, and 16 days thereafter. The DI was not suitable to determine the length of the SW. In the *Fusarium*-damaged kernels (FDK), a sharp 50% decrease was found after the 8th day. The largest reduction (above 60%) was recorded for DON at each resistance level between the 8th and 11th day. This trait showed the SW most precisely. The SW is reasonably stable in the first 8–9 days. This fits for all resistance classes. The use of four isolates significantly improved the reliability and credit of the testing. The stable eight-day long SW helps to reduce the number of inoculations. The most important trait to determine the SW is the DON reaction and not the visual symptoms.

**Keywords:** resistance expression; aggressiveness; *F. graminearum*; *F. culmorum*; isolate effect; disease index; *Fusarium*-damaged kernel; deoxynivalenol; susceptibility window; inoculation time and FHB response
