Control of Anthracnose Caused by Colletotrichum Gloeosporioides in Calcium Nitrate Foliar-Sprayed Dioscorea Rotundata Variety Amula

Abstract

Calcium nitrate foliar spray for the control of anthracnose disease caused by Colletotrichum gloeosporioides Penz, in the Nigerian white guinea yam Dioscorea rotundata variety Amula was investigated in this study. The field experiment was a randomized complete block design (RCBD) with three replicates. Seed yams of   0.5kg average weight were planted at the onset of rains in 2018. At 12 weeks after planting (12 WAP), calcium nitrate (analar grade) in distilled de-ionized water and without surfactant, at the rate 0mgl^-1 (control), 5mgl^-1 and 10mgl^-1 was sprayed, randomized per replicate. Anthracnose incidence and severity were assessed at 20WAP and 24 WAP. At 24 WAP also leaves were sampled according to treatments, processed and analysed for minerals and total phenolic compounds (TPC) according to standard methods. Minerals analysed were Calcium (Ca), Magnesium (Mg), Nitrogen (N), Phosphorus (P), Potassium (K) and Sulphur (S).

Disease incidence at 20WAP was 13.89% in the 10mgl^-1 treatment but was 0% in the 5mgl^-1­ treatment and also in the control. Disease severity score (DSS) was similar across treatments at 20WAP and was resistant score of 1. At 24WAP disease incidence in the 10 mgl^-1­­ treatment had decreased to 5.56% and was also 5.56% in the 5 mgl^-1 treatment, significantly lower than the control which had risen to 17.77%. With the increase in incidence in control plants to 17.77%, severity increased also to 1.10 and was 1.00 in each of the two calcium nitrate treatments, thus DSS at 24WAP was 1, resistant. Leaf total phenolic compounds (TPC) at 24WAP was 3.81mg GAE/gdm in the 10 mgl^-1 treatment, significantly higher than the 5mgl^-1 TPC which was 3.11mg GAE/g dm and the TPC of the control which was 2.82mg GAE/gdm. Antifungal properties of phenolic compounds in the leaves of cultivar Plimbite of Dioscorea alata is reported to be responsible for its resistance to anthracnose. Increase in leaf TPC content in treatment 10mgl^-1 in this study may be responsible for the decrease in anthracnose incidence from 13.89% to 5.56% in the treatment over the control at 24 WAP. 10 mgl^-1 Calcium nitrate foliar spray is suitable in white guinea yam variety Amula for increase in total phenolic compound content in the leaves. Leaf sulphur content was similar across the three treatments. Calcium, Magnesium, Potassium and Phosphorus were significantly higher in the two calcium nitrate treatments than the control. Calcium role in membrane form and function and in the enhancement of cell wall structural integrity has been reported and its increase with the Ca(NO₃)₂ foliar spray would reduce necrosis by the pathogen and along with increased TPC enhance resistance to anthracnose in the variety. N was higher in the leaves of the 10mgl^-1 treatment plants than the control. Disease susceptibility is reported to depend on nitrogen supply and N-supply effect on susceptibility is reported to be pathogen-specific because N increase decreases severity to facultative parasites. Also N increase in reported to support metabolic activities of host cells and to delay senescence which will favour obligate parasites but not facultative parasites such as the anthracnose pathogen Colletotrichum gloeosporioides. This is so because facultative parasites are semi-saprophytes nutritionally and depend partially on senescing cells and alternatively release toxins in order to damage/kill host plant cells. Thus increase in Calcium, Nitrogen and TPC by Calcium Nitrate foliar spray in this study is favourable for resistance to anthracnose in variety Amula and other white guinea yam varieties and a second foliar spray of the two concentrations used in this study is suggested in a future trial at 30 WAP for possible reduction of anthracnose incidence to 0%.

No surfactant was used in this study in the nutrient solution. Polysorbate 20 at 0.01% had been used as surfactant in calcium nitrate foliar spray for the control of anthracnose disease in Dioscorea species but this surfactant concentration has not been established as optimum. Surfactants are reported to stimulate nutrient penetration or  absorption at low concentrations, but affect native structure of membrane-bound proteins/stability of lipid layers at high concentrations, leading to loss of membrane integrity, implying that non-optimal surfactant concentrations in the nutrient spray would cause additional necrosis along with that caused by the anthracnose pathogen. This study shows that despite the non-addition of surfactant (polysorbate 20), the extent of absorption of calcium nitrate was enough to increase the TPC, Calcium and Nitrogen responsible for anthracnose control in the sprayed plants.