This report is the result of observations of our family orchard, from discussions and observations with many rare fruit growers in Queensland.
Of our initial planting of seven Green Sapotes, all grafted on Mamey Sapote rootstock, we have had only four survive. Originally the majority of Green Sapotes in Queensland were grafted on Mamey Sapote rootstocks.
Although the grafts on these plants successfully took, after a period of several months, severe cracking and splitting occurred, at and above the graft union.
This cracking and splitting is a very serious problem because the trees thus affected in our orchard either died or had some very serious health problems. Tests revealed that Phomopsis fungi had entered these cracks and splits. This fungi is not normally a problem.
Several factors have created the situation where it has become a major problem with tree death being the result.
1. The cause of the initial problem would appear that Mamey Sapote rootstock is not compatible with Green Sapote. Every Green Sapote I have seen grafted on Mamey rootstock shows incompatibility to some degree. Whereas those grafted on green sapote rootstock show none of this apparent incompatibility, and one is hard pressed to find the graft union after a few years. Definitely no splitting and therefore no Phomopsis fungi.
2. The growth of the fungi is ensured if under-tree sprinklers are used. As the bark splits and cracks it develops a corky texture which seems to absorb moisture and provide the fungi with ideal conditions. Although the application of copper oxychloride will prevent the fungi, it is difficult to keep it on the problem area if the graft union is low down on the trunk. The copper is continually washed off with irrigation.
Mr. Whitman of Florida has stated that if the fungi can be controlled initially, the splitting becomes less of a problem as the tree matures.
Personally, I won't consider the problem solved until I am able to approach-graft several seedling green sapote as rootstock to my surviving, ailing green sapote trees. The shortage of these as rootstocks has certainly caused problems.
DATE: July 1992
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