Research Article Open Access
Infection in Tuber Head, Middle and Tail by Rhizopus stolonifer (Ehrenb.) Lind in Relation to Calcium Content in Dioscorea rotundata Poir.
Otusanya MO*
Department of Crop protection, College of Plant Science and Crop production, Federal University of Agriculture Abeokuta, Nigeria.
*Corresponding author: Otusanya MO, Department of Crop protection, College of Plant Science and Crop production, Federal University of Agriculture Abeokuta, Nigeria; E-mail: @
Received: July 13, 2018; Accepted: August 25, 2018; Published: November 28, 2018
Citation: Otusanya MO (2018) Infection in Tuber Head, Middle and Tail by Rhizopus stolonifer (Ehrenb.) Lind in Relation to Calcium Content in Dioscorea rotundata Poir.Int J Plant Stu. 1(1): 1-4.
AbstractTop
Calcium content in tuber head, middle, tail region in four white guinea yam varieties (Dioscorea rotundata) was investigated in this study in relation to infection by Rhizopus stolonifer. This is informed by the observed higher sprouting/germination of “head” yam setts than the middle or tail. The experimental design was 4 by 3 factorial in randomized complete block arrangement of head, middle, tail per block with three replications, and variety as main plot treatment. After 4 weeks incubation, varietal mean infection of 1.32%, 1.45% and 2.74%, for varieties Oniyere, Iseosi, Efuru respectively was significantly lower than the 11.1% in variety Dakaa. There were no significant differences in infection of head (4.24%), middle (2.9%) or tail (5.33%) region, but middle region infection of 0.92% was lower in variety Oniyere and Efuru. Mean tuber region calcium content in mg/100g dry matter were not significantly different in variety Oniyere (23.33), Iseosi (21.67) and Efuru (25), but all three were higher than Dakaa (5.51). Head and middle region calcium of 6 and 6.08 mg/100g dm was lower than the 3.72 mg/100g dm of the tail in variety Dakaa. Calcium role in the plant cell wall confers structural integrity which resists pathogenic enzyme deterioration but analysis of total phenolic compounds which may confer structural integrity as well as toxic metabolites which destroy the pathogen may render explanation to the “hardy” nature of yam tuber head region compared to the middle and tail regions.
Introduction
Yams, Dioscorea species especially the white guinea yam varieties, Dioscorea rotundata Poir are probably more popular in South West Nigeria as food cultivars than the water yam, Dioscorea alata L. The water yam in South West Nigeria is popular among the Ijebu sect in Ogun state who prepare local dish refered to as "Ikokore" from the grated cooked and spiced meal. Water yam may also be peeled, grated, spiced, made into balls and fried into a meal snack called “Ojojo”. Apart from these two, preparations, the Yoruba in the South West relish various preparations from White guinea yams which outnumber those for which water yam is put. Also many whiter guinea yam varieties are available in a yam (South West Nigeria) market relative to water yam varieties. The South East Nigerian region however has numerous dishes/ preparations from water yam and relatively more water yam varieties. Yoruba farmers in South West Nigeria plant sections usually cut or prepared from whole tubers, which are refered to as yam “setts”. The setts are preferred for the smallholder farmer because ware yams are more costly for planting.

The alternative is the smaller-sized yams refered to as “seed yams” which are formed after the milk harvest (at six months after planting) when big-sized tubers are severed with sharp knives from the shoots in a yam mound. The severing is done such that a part of the proximal (head) region is left, still attached to the shoot. The latter are then buried again for the last 3 months (month 6 to 9) of the growth cycle when the severed portion bulks again to produce the small-sized yams at physiological maturity which are refered to as “seed yams”. The latter are usually expensive especially towards the onset of rains which is generally the time to plant yams. A small holder farmer buys as many seed yams as can be afforded financially and supplements with yam “setts” cut from the ware yams which normally may be as big as 2 to 6 kilogram weight. Ware yams are harvested at physiological maturity when yam leaves become senescent, generally 9 months for white guinea yam.

It is usual to have faster emergence from setts prepared from the proximal region/head region as farmers are familiar with rots affecting more setts from the distal portion of the tuber in the field. Differences in infection by Aspergillus niger from different geographical locations (Nigeria, Asia and Europe) on cut regions of the yam (Dioscorea species) tuber were not consistent probably because of a mix up of varieties from their sources (provenances) (Otusanya and Jeger, 1994).

Calcium fertilization in two improved varieties of Dioscorea spices namely D. rotundata TDr 131 and D. alata TDa 92-2 reduced infection by Aspergillus niger and Botryodiplodia theobromae after long-term storage, especially in yam sections/yam setts (Otusanya et al., 2016).

This study investigated infection in different regions namely head, middle and tail of tubers of four varieties of white guinea yam from South West Nigeria, by Rhizopus stolonifer, an important yam tuber rot pathogen. Calcium content of the head, middle and tail of tubers of the four varieties were also analysed to ascertain relationship between it and infection.
Materials and Methods
Yam, Dioscorea species tubers
Tubers of four varieties of white guinea yam Dioscorea rotundata were sourced from the yam farmers’ market in Gbonogun main market, Abeokuta, Ogun State in South West Nigeria. They are varieties Efuru, Oniyere, Iseosi and Dakaa.
Analysis of Tuber Calcium
Three tubers per variety were selected for the tuber calcium analysis. The selected tubers were free of abrasions or injuries or disease symptoms. They were washed in running tap water without bruising them and left to dry on top of a laboratory bench under a low-speed fan. Each tuber was then cut into sections of head, middle and tail. Each section was sliced thereafter into thin chips inside labelled plastic trays. Drying of the chips was for three days on yam storage structures inside the COLPLANT (College of Plant Science and Crop Production), Federal University of Agriculture, Abeokuta (FUNAAB), Screen house. The dried chips were ground to powder with a high powered mill at the Central Workshop of COLENG (College of Agricultural Engineering), FUNAAB. They were then poured into new labelled polyethylene bags for calcium content analysis. Analysis of calcium content was carried out with the routine methods of mineral (Calcium) analysis of the Association of Official Analytical Chemists [1].
Infection and weight loss experimental design and procedure
Tubers which had no bruises, lacerations or disease rot symptoms (soft, dry or wet) of the four varieties were selected. They were cut into fairly equal regions of the head, middle, and tail perpendicular to the tuber length, after the head and tail tips had been cut off, with a sharp surface-sterilized steel knife [3]. They were left to dry for 24 hours on top of surface-sterilized Laboratory benches in the Crop Protection Laboratory of the Department of Crop Protection, COLPLANT, FUNAAB, Abeokuta, Ogun State in the South West area of Nigeria. Each of these cut regions of head, middle and tail from each tuber had two cut surfaces and served as the experimental units [4]. Each yam section was labeled and weighed with an electronic balance at the beginning of the experiment. Cut sections of the four varieties were then set up, in a 4 x 3 factorial experiment, arranged in randomized complete block, each of head, middle and tail, and three replications.

Each experimental unit was inoculated with a 7-day old pure culture of Rhizopus stolonifer which had earlier been isolated from a partially rotted tuber of variety Efuru. Inoculation of experimental units was according to the method of Otusanya and Jeger (1994). Surface sterilized 4 mm and 3 mm cork borers, scalpel and forceps were used to bore 10 to 12 mm hole into each unit and to place a 3 mm agar (potato dextrose agar) disc of the pathogen into the hole. After which the upper incision on the periderm was sealed with Vaseline (petroleum jelly). The inoculated sections were transferred in trays into a raised wooden netted/ventilated yam storage structure inside the COLPLANT screen house. The yam storage structures are protected from rain, rodents/reptiles and allow free air flow which disallows increase of humidity. Incubation period was 4 weeks. Each section was weighed again with an electronic balance after the 4 week incubation. Then, each yam section was cut open with a knife, after the Vaseline had been removed with spatula and cotton wool. Infected tissue was cut into a pre-weighed petri-dish and its weight measured with an electronic balance. Percent infection was determined with the formula:

Where C and A are corrected weight of infected tissue and weight of the yam section at the beginning of the experiment respectively. C was calculated with the formula: $\text{C}=\frac{100\text{X}}{100-\text{Y}};$ where X and Y are the weight of infected tissue and percentage weight loss respectively. Percent weight loss was calculated with the formula: , where A and B are weight of the section at the beginning and at the end of the experiment respectively.
Data Analysis
Percent data was transformed appropriately before Analysis of variance. Means were separated with Tukey’s (HSD) test.
Results
Infection in tuber head, middle and tail and calcium content