PRIMOCANE-FRUITING BLACKBERRY PERFORMANCE IN HIGH TUNNELS IN COLD REGIONS

Six primocane fruiting blackberry (Rubus L. subgenus Rubus, Watson) genotypes (‘PrimeArk-45’, ‘Prime-Jan’, ‘Prime-Jim’, and three advanced selections from the University of Arkansas, USA) were planted in a multi-bay high tunnel in Michigan in 2007 to compare productivity and fruit quality. Plastic was placed on the tunnel in May and removed in early November. Sides and ends were covered with plastic after early-mid October to retain heat. Canes were pruned to the ground in March. In 2008, primocanes were pinched (terminal 5-10 cm removed) as they reached heights of 80 cm, and lateral branches were also pinched if they grew longer than 20 cm. No pinching was done after 15 July. Plants began producing fruit in mid September, but overall yields were very low. In 2009 and 2010, primocanes that reached 80 cm before July 1 were pinched, and canes reaching this height later as well as lateral branches, were left alone. Fruit matured from early August until the middle of October, with no period of concentrated ripening. Total fruit yields were generally equivalent to 1-3 t.ha^-1. ‘Prime-Jan’ and ‘Prime-Jim’ tended to be the most productive types and ‘PrimeArk-45’ produced the largest berries. Crown gall (Agrobacterium tumefaciens) was observed on plants and may have reduced yields. There was evidence of poor fruit set that may have resulted from high temperatures in July and August. However, yields were also low because many canes did not flower or flowered too late to mature fruit. In 2009 and 2010, 20 and 60% of canes and branch terminals had not produced ripe fruit by the end of the season.