Leaf spot (also called brown leaf spot) of turmeric (Curcuma longa) occurs throughout in our country. It is prevalent along the coastal areas of Orissa and Andhra Pradesh and epidemics of the disease have been reported from these areas.
The disease causes considerable damage in turmeric-bowing areas of Tamil Nadu and Gujarat. In northern India the disease appears quite commonly and in some areas almost 100% leaves of each infected plant are affected during August and September. However, a few other species of Curcuma and some of Zingiber are also attacked by the same pathogen.
Symptoms of Leaf Spot Disease:
The infected leaves turn from normal green to finally reddish-brown due to great numbers of spots covering both the surface of the leaves. Individual spots are small, light yellow to dark yellow, 1-2 mm in diameter. The spots are comparatively more prominent on the upper surface of the leaf, fuse with each other farming larger patches causing the drying up of the entire lamina. The leaves turn from green to yellow much sooner than they normally should but are not distorted due to the disease.
Attacked plants are not killed but due to excessive spotting and reduction of the green tissue the functional area of the lamina is reduced resulting in heavy reduction in the yield. The pathogen does not stimulate any outgrowth (hypertrophy and hyperplasia) on the infected host tissue as happens in case of peach leaf curl caused by Taphrina deformans.
Causal Organism of Leaf Spot Disease:
The pathogen is a biotroph. Its hyphae spread through the space between the cuticle and the epidermis and are present inside the epidermal cells. Below epidermis, the hyphae invade through intercellular spaces and send branched or lobed haustoria within the host cells but their invasion is limited to few layers of host cells below the epidermis. When the spots are mature, their central portion is occupied by an almost continuous layer of hyphae.
The outer cells of this layer act as acrogenous cells and develop into cylindrical or clavate, thin-walled projections, which rupture the epidermis and become asci. Since the acrogenous cells mature at different intervals, the mature asci containing ascospores are evident in little groups.
Each ascus possesses a basal cell below it. However, the asci measure 20-30 x 6-10 µm and normally contain eight ascospores. Each ascospore is hyaline, avoid, unicellular measuring 4-7 x 2-3 µm, and multiplies by budding within the ascus to form sprout conidia.
Leaf Spot Disease Cycle in Turmeric:
The disease cycle is yet not clearly known. How pathogen is transmitted is still speculative. How long the ascospores remain viable in plant debris has not yet been ascertained.
The possibility that the mycelium approaches the rhizome and perennates there in and then acts as source of primary inoculum when such rhizomes are planted the following season is remote. It is thought that, most probably, the infected fallen dried leaves in the soil possessing asci might be functioning as the main source of primary inoculum during the subsequent crop season.
Management of Leaf Spot Disease:
(i) Although no definite measures to manage the disease have so far been developed, some cultural practices are recommended for the purpose. Field sanitation involving destruction of the diseased fallen dried leaves should be adopted.
(ii) Timely sprays of Bordeaux mixture (5:5: 50). Fytolon, Perenox, Blitox 50, and Dithane Z-78 have been found effective in reducing the spread of the disease to some extent.
(ii) Varieties like ‘China’ and ‘Jaweli’ of turmeric (C. longa) and the species of C. armada show resistance to the disease hence recommended for use. In Assam, cultivars Ca 69 and Shillong are resistant to the leaf spot disease of turmeric.
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