Articles
APPLE DECLINE IN BRAZIL
The primary symptoms are low vigor, lack of terminal growth, absence of feeder roots, and die back.
Frequently leaves near the terminals are small and chlorotic similar to zinc deficiency.
In addition, fruit production is markedly reduced and the fruit is small.
The central leader is rubbery and dies back.
Virus indexing revealed the presence of several latent viruses including rubbery wood.
As a result of the symptoms, the virus indexing, and the brown color of the wood in the autumn, this type of tree decline has been attributed to rubbery wood.
The symptoms are strongest in ‘Gala’ apple on MM 106 rootstock; however, other cultivars such as ‘Fuji’, ‘Golden Delicious’, ‘Melrose’, and ‘Granny Smith’ also express the same symptoms only milder.
The symptoms appear stronger on MM 106 rootstocks than on other rootstocks such as M 7 or MM 111.
Roots from symptomatic trees were collected and fixed in 5% glutaraldehyde.
DAPI tests on root sections, as described by Seemuller (1976), revealed fluorescent particles in the phloem seive tubes.
In subsequent tests, a 100% correlation was observed between the DAPI-positive particles and declining apple trees in Brazil.
In May 1990, in the autumn after harvest and before dormancy, oxytetracycline was injected, as described by Parish (1989), into declining trees in several orchards in the Fraiburgo, SC area.
This growing season (1990–91) was extremely unusual in that it was droughty.
None of the trees grew well and it was not possible to fully evaluate the results of the oxytetracycline injections.
However, it was obvious the roots of the trees injected with oxytetracycline were more vigorous and had more feeder roots than roots on the control trees.
These results indicate the possibility of a mycoplasma-like organism being associated with declining apple trees in Brazil.
Roots from diseased trees in Brazil were collected, and total DNA was extracted using a procedure described by Dellaporta et al. (1983). Using a modification of a technique described by Kollar et al. (1990), a MLO-enriched DNA fraction was prepared using cesium chloride/bisbenzimide gradients.
This MLO-enriched DNA faction will be cloned and screened as a diagnostic probe.