Tolerance to BYDV: Factors influencing reactions of plants on infections

All plants of cereals and at least most of other grasses are susceptible ro Barley yellow dwarf virus (BYDV). Because susceptible plants which are res istant tolerate infections by BYDV. If already stressed by pathogen infecti ons, growth and development of infected plants are additionally influenced by further stressing agents. Standardized methods were used to study the in fluence of different Factors on reactions of plants after infections by BYD V. Results show that environmental conditions predominantly are stressing f actors. Therefore, dependent on changing climatic conditions, the economica l value, such as kernel yield or dry matter, which was used as criterion fo r evaluating tolerance, varied depending on the geographical location of ex perimental sires. Furthermore, it could be demonstrated that, using differe nt isolates of BYDV-PAV, tolerant or non-tolerant reactions of plants depen d on the relationship between the degree of 'metabolic power' of the host a nd the pathogen-specific 'aggressivity'. Plants are tolerant only if they c an counteract the 'aggressivity' of the pathogen. Results of investigations presented here support former findings that tolerance can only be reliably estimated when plants become infected during early stages of development. Because of difficulties in assessing tolerance under often unpredictable na tural test conditions, the use of standardized methods is recommended for s election of world-wide durable tolerance to BYDV. By using those standardiz ed rest methods, obtained results of our investigations lead to doubts on t he function of the "Yd2" gene supposed to be responsible for governing the tolerance to BYDV.