The first signs of imminent death are that the tree's growth atrophies and its leaves turn pale yellow or reddish. It usually starts in a single branch. When that happens, producers know that the tree will collapse in a few weeks. The phenomenon is known as rapid apple decline (RAD) and it is causing real concern among producers, especially in the northeastern United States.
It doesn't affect only one tree at the time. When one of the trees becomes ill, the neighboring apple trees of the same garden immediately begin to show the same symptoms. Scientists have been observing similar phenomena for some time, especially since the 1980s, but the most serious outbreak of this infection, if it can be called that, occurred in 2013.
Back then, pathologist Kari Peter of Pennsylvania State University discovered a massive and inexplicable death of young apple trees in one of her research orchards. Scientists used a variety of chemicals to keep it from spreading, but nothing worked. The symptoms did not coincide with any pathogen that usually affects trees.
Soon after, there were other similar cases in Pennsylvania orchards and, since then, the RAD phenomenon has broken into several apple-growing areas in the central, northeastern, and northwestern parts of the United States, and also in Canada. In North Carolina up to 80% of the plantations show symptoms of this lethal disease. Rows and rows of trees are collapsing like playing cards, according to Peter.
The confusion is greater because there is no clear factor that can be behind this problem and the behavior of the trees themselves is quite confusing. Sometimes it spreads to its neighbors and sometimes it does not; sometimes the apple tree looks bad, but it turns out to be fine. Nobody knows if it is an endemic disease or if it is caused by other stress factors. Plant pathologist Awais Khan, of Cornell University, analyzed soil nutrition and climatic conditions of lands affected by RAD for five years in order to determine which viruses, fungi, or bacterial communities might be causing this disorder.
In his conclusions, published in a document in PLOS One, Khan can only suggest that climate stress and limited access to water could make apple trees more vulnerable to something that is still unidentified. "We did not find statistically significant differences in the soil and climate profiles of healthy-looking and declining trees," the study states. Nor was it linked to fungi or viruses.
The causal agent is still unidentified. The incompatibility between the crop and the rhizome, the extreme climatic conditions, the insects that perforate the wood and the infection by pathogens have been proposed as possible causes. The trees that present incompatible grafts show breaks or malformations in the union of the graft, chlorosis of the leaf, early defoliation, wilting of the plant and premature death.
Researchers have also suggested that unfavorable weather conditions are involved in RAD. For example, frost can cause direct damage to the plant tissue, making it vulnerable to secondary abiotic or biotic stress. In the same way, drought or flood can cause a delay in the growth of buds and leaves, chlorosis and leaf defoliation, root necrosis, wilting and, finally, senescence of plants.
More than an insect
Insects that pierce wood can also cause serious damage or death to apple trees. Insect-infested trees generally have a sickly appearance, sparse, pale-colored foliage, and can die with a heavy fruit crop during the ripening stage of the fruit. The involvement of plant pathogens in RAD is still a matter of speculation.
Peter suspects that it is something more serious than an insect or a pathogen. The apples is the most consumed fruit in the North American continent and it is cultivated in 32 states with harvests that overcome the 4 billion dollars.