I - --ii

Proceedings ofthe Workshop on Ascachyta Blight and Winter Sowing of Chickpeas (Saxena. M.C.
and Singh, K.B., eds.), ICARDA, 4-7 May 1981, Aleppo. Syria
International Program on Horizontal Resistance
to Ascochyta Blight
R. PIETERS
40 Casier ONU, Rabat-Chellah, Morocco
Although chickpea in Morocco has relatively few important diseases and pests,
Ascochyta rabieican cause 100% crop loss when conditions are favourable to the
development of the disease. Late-sown chickpea can escape the disease but tie
drop in yield due to late sowing does not make this procedure very acceptable. To
solve this problem, a breeding program has been started to develop agronomically
acceptable cultivars wOth stable resistance to ascochyta blight. The main
difference between the International Program on Horizontal Resistance (IPHR)
and a conventional breeding program is the type of resistance used. The polygenic
nature of the resistance and its quantitative expression require a specific
approach. In this paper the horizontal resistance breeding program is discussed
and its progress evaluated.
Horizontal - Vertical Resistance
The terms horizontal and vertical resistance originate from van der Planck
(1968) who described two types of resistance which can be defined as follows:
Vertical Resistance Horizontal Resistance
Gene for gene action No gene for gene action
Qualitative Quantitative
Monogenic Polygenic
Affects onset of epidemic Affects development of epidemic
In agricultural history, horizontal type resistance (HR) has generally remained
more stable than vertical type resistance (VR). Although VR can be
made more durable by proper management, in host-pathogen systems in which
the pathogen is highly flexible, HR should be preferred. HR and VR have been
described as LNo mutually excluding phenomena; in practice, however, the dif­
89
Previous Page Blank