**3.3 Symptomatology and diagnosis**

Proper and early disease diagnosis is vital for applying control practices at the right moment. Without an efficient and effective early diagnosis of the disease and the disease-causing agent, any control measures will be inefficient [43]. Until the FY etiology is known and diagnostic systems developed, the only way to find out that a plant has this disease is by checking for characteristic symptoms and signs. Once a plant is diagnosed with FY, it must undergo roguing. In Brazil, symptoms identification in the field is still the only diagnostic system used for FY [8, 12].

An oil palm plant affected by FY shows necrosis or dryness of the spear leaf that evolves towards the base, then the region of the meristem rots, and a foul odor is felt in some cases (**Figure 2**) [12, 44]. The process of rotting of the meristem region, frequently observed in rainy seasons, motivated the initial designation of the disease as spear leaf rot [8, 40]. After losing the spear leaf, there is a general decline leading to plant death; however, some individuals during this process may temporarily re-issue a new one [12, 18]. In plants affected by FY, chlorosis appears in leaflets at the base of the intermediate leaves, which advances towards the extremity, followed by necroses frequently observed in younger leaves [6]. There is no synchronism between the spear leaf necrosis and the chlorosis of the leaflets. The FY symptoms always begin with leaflets chlorosis, which led to the Fatal

*Oil Palm Fatal Yellowing (FY), a Disease with an Elusive Causal Agent DOI: http://dx.doi.org/10.5772/intechopen.98856*

Yellowing disease name [1]. In Brazil, the oil palm tree usually dies 7 to 10 months after the onset of the first symptoms, but it can vary depending on the region [41].

Once the oil palm plant gets affected by FY, the developed bunches can reach the maturation stage and are not affected. However, the immature ones rot, and the inflorescence abort [40, 41]. The root system is visibly affected, and emission of new primary roots reduced, leading to a total cease of roots growth. FY kills the tips of the roots generating new false primary ones. In addition, the root tissue is usually necrotic at the beginning of the appearance of symptoms in the aerial part [45, 46]. On the other hand, no apparent internal symptoms are observed, such as rot or necrosis of the stipe and vascular system, a characteristic that is also seen in PC [41].
