Background and objectives
From the beginning of this century there are references describing unexplained deaths of cork oak trees, independently of their age. In 1980s foresters became concerned with increasing patches of dead and dying trees in southern regions of Portugal . The same kind of symptoms have been observed recently on holm oak trees (Quercus rotundifolia), a very commom tree in inland areas of Alentejo and Algarve regions. In affected areas plant trees exhibit a range of symptoms and a variable rate of disease expression while others die suddenly. Those that show evidence of chronic disease show a gradual deterioration of the crown that starts with leaf chlorosis and dieback of leaf bearing branches. Some references assigned cork oak decline to biotic factors such as insects and pathogenic fungi and/or abiotic factors such as draught, waterlogging, overgrazing, etc. It is difficult to discriminate the effect of each factor on disease severity, a question that has given rise to much controversy in Portugal. Studies carried out in order to evaluate the role of Phytophthora cinnamomi on the declining of cork and holm oak trees showed that this fungus is pathogenic to both species . The objective of this study was to determine the effect of two soil-sand water contents on the susceptibility of Q. ;suber and Q. ;rotundifolia to infection by P. ;cinnamomi as a function of time.
Materials and methods
Six-month-old plants were planted on a soil-sand mixture (20% air-dry soil+80% washed sand) artificial infested with colonized seeds of Panicum ;milliaceumwithP. ;cinnamomi. Control plants were grown in the same soil-sand mixture and the inoculum was substituted by sterilized seeds of Pn. ;milliaceum. Plants were kept in the greenhouse with the temperature ranging from 24 to 32°C for 1 ;month to restart growth. After this period half of the tested and control plants were subjected to one of two irrigation regimens: (1) soil-sand mixture water content at saturation point for 5 ;days followed by 10 ;days without irrigation; (2) irrigation according to need. Every 3 ;months 12 plants from each treatment were selected at random, removed from pots and their roots washed. Severity of disease was assessed by determining the reduction of both the shoot and root biomass, of each plant, expressed as dry weight in grams. Root samples from tested and control plants were collected for P. ;cinnamomireisolation. The experiment lasted 9 ;months.
Results and conclusions
Both species show different degrees of susceptibility to infection under the two watering regimens. At saturation point the root biomass of holm oak in tested plants was significantly greater than control plants, independently of time. The cork oak plants tested showed a significantly decrease in biomass only at the end of the experiment, when compared with controls. These results suggest that Q. ;rotundifolia is invaded more quickly by the fungus thanQ. ;suber. At normal irrigation, root biomass of Q. ;rotundifolia plants tested also differ significantly from that of controls. Conversely, cork oak plants tested, under normal irrigation did not show any aerial symptoms, although the fungus was reisolated from thin roots.These results indicate that Q. rotundifoliais highly susceptible to infection by P. cinnamomiindependently of the water regimen used, while Q. ;suber showed clear symptoms of decline only at the end of the experiment on tested plants subjected to periodical waterlogging. Control plants of both species were not affected by waterlogging.
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