| 1. CERCOSPORA LEAF SPOTSymptoms:
- The characteristic symptoms include, discrete circular lesions, 3-5 mm in diameter, with a necrotic center and reddish to dark brown margin (Plate-1) and in case of severe infection, the petioles are also infected.
- The spots are scattered at first but in case of severe attack they coalesce and cover the entire leaf blade and affect the quality and yield of seeds.
- In seed crop, all the above ground parts including seed clusters are affected.
Disease cycle and epidemiology:
- The disease is caused by Cercospora beticola Sacc.
- The mycelium of the fungus is septate, dark coloured and intracellular.
- Small sclerotial masses are formed on the host tissue from which dark coloured conidiophores arise in clusters.
- The conidia are borne on the tip of the conidiophores and they are hyaline, elongated, filiform, multi-septate, broadly rounded at the point of attachment to the conidiophore and tapering slightly toward the opposite end.
- The fungus perpetuates in infected plant debris as mycelium or on the seed.
- The infected seeds result in infected seedlings.
- In the spring, overwintering mycelium in plant refuse starts producing conidia which are disseminated by wind currents, rain splashes and insects to the leaves.
- High relative humidity is an essential pre-requisite to sporulation.
- The conidial germination is best in the presence of free waters and moderate temperatures i.e. in between 15-32o C.
2. POWDERY MILDEWSymptoms:
- Collect and destroy the infected plant debris.
- Follow long crop rotations to prevent accumulation of soil borne inoculum.
- The resistance to Cercospora leaf spot has been found to be correlated with the 3-hydroxytyramine content of the leaves.
- Variety Desperzpoly RC has been reported as resistant to this disease besides giving highest tuber yield as well as sucrose content.
- Spray the crop with carbendazim or benomyl or thiophanate methyl (0.1%) and repeat at 20 days interval.
- The disease initially appears on the lower leaves and gradually spreads towards the top.
- The formation of first white, later grey-tan mildew areas on both the sides of the leaf characterize the disease (Plate-2).
- In general, infection is more on the upper surface of the leaf.
- In advanced stages of the disease development, mildew patches enlarge and coalesce and the leaf appears as if dusted with wheat flour.
- Severely affected leaves turn yellow and ultimately dry up.
- In favourable climatic conditions, cleistothecia develop as small dark round structures on the infected surface of the leaf.
Disease cycle and epidemiology:
- The disease is caused by Erysiphe betae (Vanha) Weltzien.
- Mycelium is superficial and persistent.
- Conidiophores unbranched and erect.
- Conidia ripen singly, hyaline, ovoid and 30-50 x 15-20 µm in size.
- Cleistothecia are globose, dark brown/ black, with 4-8 asci per cleistothecium.
- There are mostly 2-3 ascospores/ ascus.
- Wherever cleistothecia of the fungus are formed, these serve as the possible mode of perpetuation.
- In normal situation, the pathogen survives from one season to other in conidial state either on the perennial weeds or on other related crops grown in the area.
3. SCLEROTIUM ROOT ROTSymptoms:
- The disease can be managed by spraying wettable sulphur (0.2%), benomyl or carbendazim or thiophanate– methyl (0.1%), or hexaconazole (0.05%) or difenoconazole (0.03%) as the disease starts appearing.
- The disease is characterized by sudden rotting of mature roots below the soil surface.
- The affected root is usually covered with white mycelium that contains numerous brown sclerotia which is the most conspicuous sign of this disease.
- The fungal growth and sclerotia can be seen also in the soil around such roots.
- Later, when enough damage has been done to the roots, the leaves show yellowing and wilting and such plants can be easily pooled out.
- The pathogen responsible for this disease is Sclerotium rolfsii Sacc.
- The hyphae are hyaline, thin walled, sparsely septate when young.
- The broader hyphae show clamp connections, which are absent in thin hyphae.
- The number of nuclei/ cell is highly variable.
- Mostly there are 2 nuclei in cells of secondary and tertiary branches.
- Sclerotial initials are formed from hyphal strands that consist of 3-12 hyphae lying parallel.
- Mature sclerotia are dark brown but variation from lighter brown to darker colour may be found.
- They are small, about the size of radish seed, hard and usually round.
The basidial stage grows as a spreading white hymenium on the host surface or on the surface of the culture medium.
Disease cycle and epidemiology:
- Although the hymenia may be pure white in culture, on the host they may be grey, yellow or buff coloured.
- The basidia are obovoid. Each basidium bears 2-4 parallel or divergent sterigma, which are 2.5-4 µm long.
- Basidiospores are unicellular, elliptical to obclavate, sometimes rounded or pyriform, smooth walled, hyaline and epiculate at the base.
- The fungus is soil borne in nature and can survive as saprophyte on crop debris.
- The fungus also produces sclerotia that are left in the field and germinate under favourable weather conditions and cause infections.
- High temperature and humidity favour the disease development.
- In the plains of India, sugar beet is usually grown on ridges and this practice is likely to stimulate disease incidence because the lower leaves become covered with soil resulting in a ‘bridge’ of dead tissue which furnishes an ideal medium for initiating pathogenesis.
- Cultural practices like destruction of infected plant debris, crop rotation, deep summer ploughing, flooding, solarization during summer months etc. can be helpful in reducing the initial inoculum load in the field.
- Reduction in the incidence of root rot has also been recorded through the use of nitrogenous fertilizers including calcium nitrate, cyanamide and anhydrous ammonia, urea, ammonia sulphate and calcium ammonium.
- Drenching of ridge soil with carboxin and chloroneb at 2 and 15 kg/ 3000 l water/ha, respectively, significantly reduced root rot incidence and increased yields.
- Application of Trichoderma harzianum (40g/m2) inoculum was found effective in reducing the disease caused by S. rolfsii in sugarbeet.