Selection of genotypes resistant to or tolerant of pseudomonas syringae pv. Actinidiae (Psa) through EMS Mutagenesis

All the kiwifruit cultivars currently grown in Italy are susceptible to Pseudomonas syringae pv. Actinidiae (Psa), a bacterium which causes bacterial canker of kiwifruit. This disease compromises the survival of affected kiwifruit plants and significantly reduces their productivity. The aim of CRA-FRU kiwifruit breeding program was to induce mutagenesis in order to find genotypes tolerant of or resistant to Psa. EMS (ethyl-methane-sulfonate) at three different concentrations (0.2, 0.3, 0.4% w/w) was used on c. 10,000 seeds belonging to different Actinidia species: 3540 'Hayward' open pollinated (o.p.) seeds (A. deliciosa), 3,300 seeds of a CRA-FRU selection, RII-28 (A. chinensis), and 3,240 seeds of another A. chinensis selection, C8, kindly obtained from Udine University. More than 90% of the seeds germinated and, after 120 days, all the seedlings were inoculated with a Psa suspension of the strain CRA-FRU 8.43 (1-2×106 cfu/mL) by direct injection into the leaf. Symptoms were checked each week after inoculation. After four months surviving seedlings were inoculated again in the stem: 137 of the 'Hayward' o.p. seedlings, 65 of R-II 28 and 24 of C8 seedlings survived the second inoculation respectively. In the 2013 spring, only 5 plants of 'Hayward' o.p. treated with 0.2% EMS were still alive. In the following year, pollen was treated by holding an anther for 1 or 2 hours in a 1% EMS solution. Crosses were made using the mutagenized pollen and the 11,000 seeds collected were planted in December 2012. A very low germination rate (9%) was observed. The larger seedlings were inoculated, from the end of May to the end of June, by leaf injection and the smaller ones by irrigation with an aqueous bacterial suspension (1×106 cfu/mL). All surviving plants were analyzed by PCR to establish the presence/absence of the bacterium. An in vitro callus mutagenesis experiment is in progress in order to induce mutagenesis followed by in vitro use of Psa inoculation as selection strategy.