The apple disease fireblight was particularly bad in SW Michigan this past season. Many of the infected apple trees will be removed this fall and winter.
Depending on the cultivar and its stage of development at the time infection occurs, a single blossom or shoot infection can result in the death of an entire limb, and where the central leader or trunk of the tree is invaded, a major portion of the tree can be killed in just one season. In general, infections of any type that occur between petal fall and terminal bud set usually lead to the greatest limb and tree loss. In addition, heavily structured trees tend to suffer less severe limb loss than those trained to weaker
systems for high productivity.
Where highly susceptible apple rootstocks (M.26, M.9) become infected, much of the scion trunk and major limbs above the graft union very typically remain symptomless, while a distinct dark brown canker develops around the rootstock. As this rootstock canker girdles the tree, the upper portion shows symptoms of general decline (poor
foliage color, weak growth) by mid to late season. In some instances, the foliage of trees affected by rootstock blight develop early fall red color in late August to early September, not unlike that often associated with collar rot disease caused by a soilborne fungus. Some trees with rootstock infections may not show decline symptoms until the following spring, at which time cankers can be seen extending upward into the lower trunk
Several new rootstocks have been developed which have better resistance against fireblight. For those of you unfamiliar with apples, the rootstock is the root system of the tree. Apple varieties such as Red Delicious are budded on the rootstock. Some rootstocks influence the top to be small, some have better disease or insect resistance.
We have been watching several new rootstocks from CornellUniversity—the CG series…which stands for the Cornell/Geneva. These CG rootstocks have been developed with disease resistance against fireblight.
CG65, CG16, CG11, CG30. Trees on these rootstocks range from very dwarf to ¾ size tree.
The CG65 is probably too dwarfing for all but small backyard plantings.
GC30 is a ¾ size tree approximately the same size as trees on M.7 rootstock and much more productive than M.7. Trees on GC30 may be more brittle with varieties such as Gala and may need support.. 'GenevaTM 30' rootstock, which produces a tree about 50% the size of seedling, has continued to show good resistance to fire blight, high yield efficiency in several field plantings on growers farms and in the US national rootstock testing program. However, it has shown graft union brittleness with 'Gala'. 'G.30' also has the deficiency of producing many lateral spines on stoolbed shoots. This has limited its production by commercial stoolbed operators.
Trees on G16 are semidwarf, slightly larger than trees on the popular M.9 rootstock and are equally productive. For high density plantings, growers should order their trees on G16. 'GenevaTM 16', which produces a tree about 30% the size of seedling, has been slightly more vigorous than 'M.9NAKB337', but has been equally productive. It has shown very high resistance to fire blight. It has had excellent performance in the nursery except where virus infected scionwood has been used. Tests have shown that 'G.16' is sensitive to one or more latent viruses.
'GenevaTM 11' rootstock, which produces a tree about 40% the size of seedling, has shown very high productivity and good tree survival, but not immunity to fire blight. A field planting of 'Gala' on 'G.11' showed 25% tree loses when inoculated with fire blight while 'M.9' and 'M.26' trees showed 80'100% tree death
The Geneva program has several other rootstock in the pipeline. Look for CG.4202, CG.3041, and CG.5935 for at least a trial planting.
At the top of the list are CG.3041, CG.5202 and CG.5935.
CG.3041 will be challenging Malling 9 -- full dwarf; productive; better anchorage; fire blight resistant.
'GenevaTM 3041' and 'GenevaTM 5935' which are both fire blight resistant, extremely productive and produces trees 30% and 50% the size of seedling, respectively.
CG.5935 will challenge Malling 26; both have good fire blight resistance. Trees on CG.5935 are extraordinarily precocious and productive; we think this stock will be especially valuable under varieties like Northern Spy and Golden Russet.
CG.5202 'GenevaTM 202', which is being commercialized in New Zealand, produces a tree about 40% the size of seedling. In addition to having high fire blight resistance, it is also woolly apple aphid resistant. CG.5202 will probably be introduced for New Zealand, Brazil, South Africa and Australia, where the woolly apple aphid is a critical problem.
Again, these rootstock do not seem to change the susceptibility of the variety budded on top, but they do seem to inhibit the development of the late summer tree decline.