| 1. CORIANDER WILT
The occurrence of the disease in India has been reported from Madhya Bharat as early as 1952. In favourable weather, the disease causes significant yield reductions.
- Initial infection occurs on radicles causing brown lesions, rotting of tissues and finally pre-emergence mortality.
- In post-emergence, underground portion of hypocotyls initially develops lesions, which slowly coalesced and girdled the region leading to death of the plants.
- Adult stage symptoms are typical and characteristic of Fusarium wilt.
- The wilt symptoms are initiated by yellowing and curling of lower leaves.
- Gradually these symptoms get extended up to the apical portion, and during this period lower leaves become dry and plants droop.
- Affected plants generally remain stunted and bear less flowers and fruits.
Disease cycle and epidemiology:
- The fungus responsible for this disease is Fusarium oxysporum f.sp. coriandri Narula & Joshi.
- The mycelium is abundant, fluffy and cottony white to light pink, both micro- and macro conidia are produced.
- Later in the season chlamydospores are also produced.
- The fungus is soil borne and overwinters in the form of chlamydospores.
- In favourable weather conditions, chlamydospores germinate and cause infection.
- Microconidia and macroconidia are produced which cause secondary infections.
- Optimum temperature and pH for the disease is 28o C and 5.8 to 6.9, respectively.
- Infection increased with the increasing soil moisture content.
i) Follow long crop rotations.
ii) Collect and burn the infected plant debris
iii) Addition of oil cakes in the soil and raising of soil pH to 8.2 is also found effective in reducing the disease incidence.
iv) Coriander varieties/ lines like MP 5365 and UD 373 showed least infection to the disease and can be used in breeding programme for developing resistant varieties.
v) Treat the seed with carbendazim (0.2%) followed by sprays with carbendazim (0.1%) reduce the disease.
vi) Seed treatment with Trichoderma viride (4g/kg seed) was also found effective in reducing the disease incidence.
2. STEM GALL
It is the most important disease of this crop causing considerable yield losses.
- The disease is systemic in nature and manifests in the form of tumour like swellings on all aerial parts of the plants (Plate-1a).
- The galls are soft and fleshy when young, later becoming hard and woody as they grow old.
- The peduncle as a whole appears abnormal with outgrowths or malformation along with hypertrophied flowers and fruits (Plate-1b).
Disease cycle and epidemiology:
- Fungus responsible for this disease is Protomyces macrosporus Unger.
- Hyphae are intercellular, closely septate and broad, branching is irregular, scattered cells in the hyphae swell, form ellipsoidal or globose bodies, which later develop in to chlamydospores.
- As the chlamydospores mature, a thick, hyaline and three-layered wall measuring 50 to 60 µm in diameter surrounds them.
- The mycelium of the fungus is only found in the tumours although the resting spores of the fungus cause systemic infection.
- The disease is seed and soil borne in nature.
- In soil the fungus overwinters in the form of chlamydospores.
- The chalamydospores germinate in the presence of water by rupturing the outer wall.
- The inner wall is pushed out to form vesicle, which appears in continuation with the mouth of the crack.
- The protoplasm passes into this vesicle and the nucleus divides several times forming 100 to 200 daughter nuclei.
- The protoplasm also divides by cleavage into several uninucleate masses.
- On maturity, these spores separate and collect in the centre of the vesicle. The latter bursts and the spores are set free.
- These spores further multiply by budding in yeast like fashion and cause infection of the host.
- Minimum/maximum atmospheric temperature and relative humidity plays an important role in the initiation and development of this disease.
- Potassium and nitrogen fertilizers reduced stem gall incidence while phosphorus fertilizers increased it.
- A pH of 7.5 was most suitable for infection while minimum infection occurred at pH 5.5.
i) The disease affects less in the early and late sown crops. Sowing crop around 16 October and 16 November gave minimum yield losses. 3. POWDERY MILDEW
ii) Use of clean and healthy seed and follow suitable crop rotation.
iii) Use resistant cvs./ lines like JD 1, G-5365-91, Pant Haritma, UD 20, Rcr 41, Pant-1, CIMAP 2053.
iv) Seed treatment with captan/ thiram (0.3%) followed by their foliar sprays were found effective in managing this disease.
Powdery mildew of coriander is an important disease of this crop and in favourable dry weather conditions can cause significant yield reductions.
- The disease first appears as minute discoloured specks from which a powdery mass radiates on all sides.
- Large areas on the aerial parts of the host may be covered with white floury patches (Plate-2). The superficial mass consists of the mycelium and spores of the fungus.
- In advanced stages of attack, infected peduncles dry prematurely and render the flowers sterile or lead to poor seed setting.
Disease cycle and epidemiology:
- The disease is caused by Erysiphe polygoni DC. which is an obligate parasitic fungus.
- The mycelium is generally fine, persistent, rarely thick.
- The conidiophores arise vertically from the superficial hyphae on the host surface.
- Conidia are formed singly or in short chain, ellipsoid or ovate with vacuolated cytoplasm.
- Sometimes late in the season, but not always on coriander leaves in the field, the cleistothecia appear as sharp, black minute bodies scattered in the mycelial web.
- The ceistothecium is provided with a number of myceloid appendages.
- Usually four to eight asci are formed in each cleistothecium.
- Each ascus contains three to five ascospores.
- Wherever the cleistothecia develop on dead plant debris, these serve as the source of primary inoculum for the next season.
- Otherwise the fungus overwinters in conidial stage on different hosts in the area, which cause primary infection.
- Wind blown conidia cause secondary infections on other plants.
- Conidial germination is best in between 20-24oC.
- A fairly dry soil and heavy application of nitrogenous fertilizers tend to increase the incidence of the disease.
- Collection and destruction of infected plant debris, application of balanced doses of fertilizers and early planting reduce the overall disease severity.
- Accessions CIMAP 2053 and CIMAP 2096 showed a high degree of tolerance to powdery mildew.
- Efficacy of various fungicides like Cosan, Sultaf, Karathane WD, Elosal, Thiovit, Morocide, carbendazim and thiophanate methyl has been reported against this disease.
- Four and three application of dinocap provided satisfactory control of the disease in the late and early maturing cultivars, respectively.
- However, these could be further reduced to three and two if the sprays are timed at flowering, seed formation and full fruiting stages in the late maturing cultivars and at seed formation and fruit formation stages in the early maturing cultivars.
4. Seed Rot and Seedling Mortality
Many fungi viz., Alternaria alternata , Aspergillus candidus, A. flavus, Curvularia lunata, C. pallescens, Cladosporium oxysporum, C. herbarum, Fusarium spp., Penicillium spp. and Stachybotrys spp. from seeds of coriander and one soil inhabitant Pythium irregulare have been reported to induce seed and seedling mortality. The pre-harvest rains increased the frequency of endophytic seed mycoflora (species of Alternaria, Cladosporium and Curvularia) and reduced the germination of spores. Seed treatment with mancozeb, captan, carbendazim + captafol and captafol proved effective to some extent to achieve better crop stand.