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Glossary of Agronomic Terms |
Agronomic Management
The agronomic management of taro comprises several practices that (i) ensure optimum
utilization of land, water, energy, inputs, (ii) minimize weed competition, and (iii) reduce
disease and pest occurrence. Practices include land preparation, planting, fertilization,
weed control, irrigation, hilling, mulching, pest and disease control, and harvesting.
Andisols
Andisols comprise the bulk of agricultural lands in many areas of past and current volcanic
activity. In Hawaii, extensive areas of highly suitable Andisols for upland crops and vegetables
are found in Maui and Hawaii. In the South Pacific, Andisols occur in New Zealand, Fiji,
Western Samoa, Vanuatu, and Papua New Guinea.
Constraints
Taro is constrained by several factors including water stress, waterlogging, soil temperature,
soil acidity, nutrient deficiency, diseases and pests, improper soil and crop management, and
stiff competition from more profitable crops. Waterlogging and disease infestation prevail
in humid lowland areas while water stress, soil temperature, soil acidity, and nutrient
deficiency are likely to occur in upland areas.
Corms
The main corm in taro (Colocasia esculenta) is actually a modified stem showing rings of leaf
scars on the surface.
Corm- Characteristics
Corms vary in shape, size, color of the flesh, and protein and starch content depending
upon (i) cultivar characteristics, (ii) planting material, (i.e., corms have a flat or
conical bottom if plants are derived from either setts or cormels, respectively), (iii)
nutrient and water supply, and (iv) environment.
Cultivars
Varietal development of taro is vigorously pursued worldwide to identify progenies that are
high yielding, disease resistant, stress tolerant, and of good eating quality. These efforts
are reflected in the availability of hundreds of cultivars for several purposes and uses.
Cultivars- Worldwide
Cultivars with widespread utilization in the tropics are Balady in Egypt, Kalpao in the
Philippines, Lehua Maoli and Bun Long in Hawaii, Thamarakanan in India, Ede Ofe and Cocoindia
in Nigeria, and Samoa Hybrid and Alafua Sunrise in Fiji and Western Samoa.
Cultivars- South Pacific
More than 100 cultivars exist in Fiji, more than 60 in Western Samoa, more than 40 in
Malaysia, 300 in the Salomon Islands, and several thousands in Papua New Guinea.
Cultivars- Hawaii
Among Hawaii cultivars, the lowland cultivar Lehua Maoli is an excellent poi taro while
the upland cultivar Bun Long is a good table taro.
Cultivars- Samoa
Samoa Hybrid outyields the best local cultivars in Fiji and has an eating quality comparable
to the cultivar Samoa16. Alafua Sunrise, in turn, is high yielding but of lower eating quality.
Cultivated Area
The world cultivated area during 1986 comprised 943,000 ha, of which 731,000 ha were located
in Africa, 162,000 in Asia, and 43,000 ha in the Pacific Basin.
Diseases
Field and storage diseases severely constrain taro production. Yield losses due to leaf
and corm rot diseases can be as high as 40 to 90%.
Diseases- Corm Rot in Storage
Corm rot is a disease that causes a rapid breakdown of the corm flesh. If improperly handled,
corms rot and become unfit for human consumption after 1 or 2 weeks of storage. In some
instances, corms begin to rot shortly after harvest and become completely decayed within 5 days.
Diseases- Corm Rot Organisms
Several organisms, i.e., dry rots (Aspergillus niger and Fusarium
solani)and soft rots
(Botryodiplodia theobromae, Rhizopus stolonifer, and Sclerotium rolfsii),
are responsible
for corm rot in storage with evidences of strong correspondence between field rot and rot
storage causing organisms. Most rot causing organisms are soil borne and are present on the
surface of corms and cormels at harvest. Thus any injury during and after harvest provides
points of entry for diseases.
Diseases- Leaf Blight
Leaf blight (Phytophthora colocasiae), a disease that causes a rapid breakdown of leaf tissues,
is known to cause severe corm yield losses, as much as 40 to 60% in the Pacific Basin and up
to 52% or more in India.
Diseases- Leaf Blight Occurrence
Leaf blight may occur throughout the rainy season on cloudy days when night and day
temperatures are 20 to 22 oC and 25 to 28 oC, respectively. The disease is particularly
virulent in high rainfall areas or under flooded conditions, particularly at high plant
populations.
Diseases- Leaf Blight Symptoms
Leaf blight is characterized by an initial formation of purplish to brownish leaf spots
that enlarge and coalesce to form larger irregular spots with a clear yellowish brown exhudation.
As the disease progresses, the whole leaf blade rots and the infestation spreads to the
petioles and corms.
Diseases- Leaf Spots
Leaf spots, produced by Phyllosticta colocasiicola and Cladosporium
colocasie, are two
benign diseases common in both upland and lowland taro. The damage is usually not severe
enough to warrant control measures other than standard sanitation measures.
Diseases- Pythium Rot
Pythium rot (Pythium spp.) is a disease that infests the corm in lowland taro. The disease
appears to proliferate in fields where water becomes stagnant and warm or under poor
field sanitation conditions.
Diseases- Pythium Rot Symptoms
Under infestation, the normally firm flesh of the corm is transformed into a soft, mushy,
often maladorous mass. The root system is virtually destroyed, plants become stunted,
petioles are shortened and leaf blades are curled, crinkled, yellowish, and spotted.
Diseases- Viruses
Viruses cause severe yield losses in highly infested fields. In Hawaii, taro progenies
are particularly susceptible to Dasheen Mosaic Virus (DMV), with Lehua Maoli showing
more tolerance than others In Western Samoa, Alafua Sunrise has shown remarkable tolerance
to DMV compared to the popular but highly susceptible Niue.
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