PhD

Yellowing of arecanut is the most serious problem affecting arecanut cultivation. The incidence of yellowing is noticed in isolated patches without any definite pattern. The diagnostic symptoms first appear as yellowing of leaflets in two to three leaves of outermost whorl, with abrupt demarcation between yellow and green regions. Yield in affected palms gets reduced by 30-40 per cent every year and in 3-4 years the entire garden becomes most uneconomical when left without proper management.

Experiments of the research project entitled “Nutritional management of yellowing in arecanut” were conducted for three years from October 2003 to September 2006. Three hundred palms, hundred each in toposequences viz. converted paddy field, garden land and terraced upland of Thrissur, Palakkad and Malappuram districts were studied to develop an yellowing index to quantify yellowing in arecanut. The incidence of yellowing among various toposequences and popular cultivated arecanut varieties were recorded. To develop a management strategy to contain the yellowing, field experiments were laid out in three toposequences viz. converted paddy field, garden land and terraced upland respectively in the farmers’ fields of Thrissur district in RBD using single plant plot with five replications. Treatments involved provision of deep drainage, application of organic manure, lime, sand, sodium silicate, magnesium sulphate, zinc sulphate, borax, ammonium sulphate, urea and three levels of potassium.

The index earlier developed by George et al. (1980) was modified as Yellowing index ( I ) = {(Y+N)/L + R} x 10, where Y is the total score for yellowing, N is the total score for necrosis, R is the score for reduction in crown size and L is half the number of leaves in crown. The modified yellowing index utilizes 13 classes in place of earlier 8 classes to score yellowing, 5 classes in place of earlier 3 classes to score necrosis and 5 classes in place of earlier 3 classes to score reduction in crown size, thus enabling precise quantification of yellowing. The highly significant negative correlation of the leaf elemental composition, growth and yield parameters with the yellowing index of experimental arecanut palm in all the three toposequences viz. converted paddy field, garden land and terraced upland clearly validated the reliability of the index.
The survey done in Thrissur, Palakkad and Malappuram district showed that highest percentage of healthy palms are existing in garden land situations. Among the three toposequences, garden land was found to be inherently more productive and supportive to arecanut cultivation than converted paddy field and terraced upland due to its physiograhic layout. The severity of the yellowing incidence was more in terraced uplands followed by converted paddy field possibly due to the imbalanced availability and uptake of nutrient elements aggravated by excessive leaching of nutrient elements in terraced uplands, and due to impeded drainage in converted paddy field. Survey also showed that local cultivars remained healthier than any of the high yielding varieties such as Kasargode local, Mangala, Mohitnagar and Sumangala as local cultivars recorded the lowest incidence of yellowing. The local cultivars being comparatively healthy could be selected and used as elite mother palms for replanting in severely yellowing affected areas.

Incidence of root rotting was found mainly confined to 16 percent of yellowing affected palms grown in converted paddy field alone. Presence of well drained soils in garden land and terraced upland might have resulted in absence of root rotting in arecanut palms grown under these toposequences. Incidence of both root xylem blockage and presence of phytoplasma in root phloem were found to occur simultaneously in the same yellowing affected palms. However these two phenomena were also found to occur independently of each other in yellowing affected palms irrespective of the toposequences under which the palms were cultivated.

In the converted paddy situation, provision of deep drainage and additional lime application have created a condition congenial for balanced uptake of nutrients as shown by enhanced calcium, magnesium and sulphur contents and reduced phosphorus contents in leaf. The deep drainage treatment increased the nut weight and kernel weight by 3.5g to 1.5g respectively. This increase has helped to maintain the yield even with a reduction in number of nuts palms-1 occurred over the years in unfertilized treatments. Application of fertilizers at 100:40:140g palm-1 in conjunction with deep drainage and additional lime application increased the chali yield by 1001 to 1138 kg ha-1. Sulphur application along with this has further increased the yield to the level of 1289 to 1412 kg ha-1 under converted paddy field. Changing the N:K ratio to supply nutrients at 100:40:200 g NPK palm-1 increased the yield by 1138 to 1344 kg ha-1. S application together with this enhanced rate further increased the yield by 1385 to 1590 kg ha-1 under converted paddy field. Enhanced rate of potassium, combined with magnesium and sulphur application resulted in the highest chali yield of 3.22 kg palm-1 (4415 kg ha-1). This treatment resulted in the least yellowing index and highest content of chlorophyll, nitrogen, sulphur and magnesium in the plant under converted paddy field.

Under garden land situation, additional application of organic manure by 5 kg ha-1 alone has not reflected in the yield increase but has improved the leaf elemental composition particularly that of nitrogen, phosphorus, sulphur and magnesium. Additional quantity of lime over farmers’ practise has helped to increase the soil pH and thereby availability and content of calcium in plants. Under garden land situation, fertilizer application at 100:40:140 g ha-1 increased the yield by 1069 kg ha-1. Additional organic matter @5 kg palm-1 increased the yield by 1316 kg ha-1 over pre experiment yield. Enhancement of N:K ratio to 1:2 or 1:2.5 without sulphur application did not increase the yield. However together with sulphur application, notable yield increase by 1289 kg h-1 at 1:2 and by 1412 kg ha-1 at 1:2.5 ratio was resulted under garden land. Highest yield of 3.99 kg palm-1 (5470 kg ha-1) which was 1604 kg more than pre application of treatments was resulted from the treatment which received 100:40:250 g NPK combined with 60 g of magnesium sulphate. Nitrogen application through ammonium sulphate was better than urea since it contained sulphur. Boron application as 20 g borax has also increased yield by 0.97 kg palm-1 resulting in 5265 kg ha-1 which was 1330 kg more than pre application of treatments under garden land.

Under terraced upland, nutrient application through sulphur containing fertilizers have found to increase the yield considerably. Fertilizer application at 100:40:250 g NPK palm-1 with 20 g borax was found to give 3.44 kg palm-1 (4716 kg ha-1) and with 20 g zinc sulphate gave 3.38 kg palm-1 (4634 kg ha-1). Application of magnesium sulphate has significantly reduced the yellowing index and increased the nitrogen, phosphorus, potassium, calcium, magnesium, chlorophyll-a, chlorophyll-b and total chlorophyll which in turn resulted in high dry matter production.