Recovery of oil palms (Elaeis guineensis Jacq.) affected by spear rots

Spear rot or PC (for the Spanish name pudrición del cogollo), known locally in Costa Rica as ‘flecha seca’ is a disorder that affects the development, physiology and yield of the oil palm. It is associated with the deterioration of the fine root system under unfavorable weather and soil conditions. The disorder is not lethal and most palms can overcome the symptoms after a variable period. The hypothesis tested in this work was that it is possible to reduce the severity of the symptoms and eventually the recovery period, using practices that improve the physical and microbiological conditions of the soil to stimulate root growth and improve the nutritional and energy status of the palms. The response obtained was measured using data on morphology, growth, physiology and yield of treated and control palms. In addition, the physical, chemical and microbiological properties of the soil were characterized. To reduce the genetic effect, the study was done with a particular clone that was known to be highly susceptibility to the disorder and that required a long period to recover from the symptoms. The study began when the palms were 28 months old and continued for two years. Two edaphic units (Coarse, Aquic Eutrudept and Medial, Aquic Eutrudept) were identified at the experimental site; where the terrain was irregular, the soil had poor drainage in the first 30 cm, high bulk density, low hydraulic conductivity and few air spaces. All these physical properties favored the presence of standing water during prolonged periods. At the end of the rainy season of 2012, a deterioration of the fine root system was observed when the soil volumetric water content increased above 40%. Imbalances were found between some nutritional elements and in the physiology of the affected palms and later on appeared the typical aerial symptoms of PC: chlorosis and drying of the leaflets at the base of the new leaves and other drying and decay of the petiole bases. The palms that received the recovery treatments showed a remission of symptoms in approximately 18 months, while the control palms were still showing major PC symptoms after two years. Visual (vegetative) recovery was associated with the remediation of the physical, chemical and microbiological properties of the soil, which in turn improved root mass and root health. The nutrient content in the leaves (N, Fe, S and K), root density, trunk height, photosynthesis, fruit production and bunch oil content increased significantly in the recovered palms compared to the untreated controls. Nutrient application (to the soil and the aerial part), and surgical removal of affected tissues in particular, helped to obtain more rapid recovery. It was evident that the removal of necrotic tissue from the petiole bases extended the longevity of the leaves. Nevertheless, the effect sought with the systematic removal of the inflorescences (for the purpose of diverting assimilates to the roots) was not clear. We conclude that the recovery from PC should focus on the regeneration and maintenance of a healthy root system and that the possibilities for success increase when symptomatic palms are treated early. However, emphasis should be placed on prevention, which means taking prompt measures to minimize the negative effects of brusque changes in environmental conditions that could stress the plants. In addition, there should be early detection and immediate correction of any factor or factors (primarily physical, chemical and microbiological aspects of the soil) that could negatively affect the maintenance of a healthy root system.