Prabhu Dayal Meena

Epidemiology and development of forecasting models for White rust of Brassica juncea in India

Experiments were laid out at Bharatpur, New Delhi and Kangra with Indian mustard (Brassica juncea) cvs ‘Varuna’ and an important one in the locality sown on 10 dates at weekly intervals. First appearance of white rust disease (Albugo candida) on leaves and pods (staghead formation) of mustard occurred between 36 and 131 days after sowing (d.a.s.), 60 and 123 d.a.s., respectively. Severity of white rust disease on leaves was favoured by >40% afternoon (minimum) relative humidity (RH), >97% morning (maximum) RH and 16–24°C maximum daily temperature. Staghead formation was significantly and positively influenced by 20–29°C maximum daily temperature and further aided by >12°C minimum daily temperature and >97% morning (maximum) RH. Regional and cultivar specific models devised could predict, at a few weeks after sowing, the crop age at which white rust first appeared on the leaves, as staghead, the highest rust severity on leaves, staghead numbers and the crop age at peak rust severity on leaf, highest staghead numbers at least 1 week ahead of first appearance of the disease on the crop.

Genetic diversity and morphological variability of Sclerotinia sclerotiorum isolates of oilseed Brassica in India

Sclerotinia stem rot caused by Sclerotinia sclerotiorum (Lib.) de Bary is one of the most serious disease and causes damage to oilseed Brassica. Seventeen geographical isolates of S. sclerotiorum from different geographical locations of India were compared for their growth under artificial culture conditions and the result was validated through random amplified polymorphic DNA (RAPD) markers. Based on the morphological studies, the isolates were grouped into fast growing (SR-01, SR-02, SR-05, SR-16) and intermediate (SR-03, SR-04, SR-07, SR-10 to SR-13, SR-18), though the rest of the isolates showed comparatively a slow mycelial growth rate. In RAPD analysis for their genetic diversity, fifty decamer primers of arbitrary sequences produced 692 scorable amplicons ranging from 180 to 3900 bp in size. Out of these, 385 fractionated fragments were reported polymorphic. As an average, 13-14 unambiguous and reproducible amplified products were generated by single primer (minimum of 4 and maximum of 23 fragments). Hence, the present investigation confirmed the diversity among the different geographical isolates from India.

Area under disease progress curve and apparent infection rate of Alternaria blight disease of Indian mustard (Brassica juncea) at different plant age

Plant age has a major influence on the incidence of Alternaria blight disease in Indian mustard crops. Disease progression was monitored twice a week on the two chosen Indian mustard cultivars viz., Varuna and Rohini throughout the season. Severity of blight caused by Alternaria brassicae and Alternaria brassicicola decreased with delay in sowing. Calculation for A-value (Area under disease progress curve – AUDPC) and r-value (apparent infection rate) in crops sown on different dates could identify the speed of progress in the disease on leaves and pods, as the crop does not posses resistance to the pathogen till date. Thus, the probable dates of sowing enabling slow disease progress or the weather conditions coinciding with the different crop phenological stages demarcated the advantageous dates of sowing from the disadvantageous ones. However, cultivar Varuna is more susceptible as compared to the other cultivar Rohini, as apparent infection rate both on leaves and pods was higher in former. Highest per cent disease severity (PDS) for season highly correlated with date of sowing, i.e. delayed date of sowing increased PDS.

RAPID SCREENING TECHNIQUE FOR ALTERNARIA BLIGHT RESISTANCE IN INDIAN MUSTARD (Brassica juncea L.) USING COTYLEDONARY LEAF METHOD

Alternaria blight (AB) caused by Alternaria brassicae (Berk.) Sacc. is a devastating disease of oilseed Brassicas all over the world, and responsible for significant seed yield losses up to 47%. No reliable, resistant germplasm is available to develop AB resistant cultivars. Various screening techniques have been reported so far, but cotyledonary leaf method is not yet reported. Three methods were tested using one susceptible cultivar (Varuna): inoculation of seed, inoculation of cotyledons, and inoculation of both seed and cotyledons. Fungal conidia were inoculated directly onto the seedlings with 1.5x105, 2.5x105, 4x105 and 5x105 conidia ml-1 concentrations for standardization. Percentage AB severity increased with the increase in conidial concentration, therefore the highest concentration was used for final screening. Among the three screening methods, inoculation of both seed, and cotyledon method was found highly effective where mean AB severity on cotyledon was 84.6% in comparison to 49.3% in the inoculation of seed and 62.5% in the inoculation of cotyledon methods. The technique was validated by screening susceptible and putative tolerant genotypes. The severity of AB was 54% of susceptible cultivar and 16.4%-21.2% of tolerant genotypes. The conidia number per microscopic field was 21.5 in putative tolerant and 43.5 in susceptible genotypes. Thus, in vitro screening of AB using inoculation of both seed and cotyledon method was found most effective and could be used for rapid screening in early stages of plant growth. A new 0-7 rating scale was also devised to observe the AB pathogen interaction phenotype at the cotyledonary stage of oilseed Brassica.

Forewarning models for Sclerotinia rot (Sclerotinia sclerotiorum) in Indian mustard (Brassica juncea L.)

Sclerotinia sclerotiorum (Lib.) de Bary has worldwide distribution and causes diseases in more than 500 host plants. Sclerotinia rot is a menace to cultivation of oilseed Brassica crops worldwide. The epidemiology of Sclerotinia rot (SR) of Indian mustard (Brassica juncea L.) was investigated during 2004-2012 crop seasons, and based on 8 year of disease data. The forecasting models were developed first time in Indian conditions and then validated in 2012-13. The carpogenic infection initiated in 52 standard week (last week of December) and continued during 1 to 3 standard weeks (first three weeks of January). Disease first appeared after closure of the crop canopy when flowering started. During epidemics, the 8 year mean daily maximum and minimum air temperature was 19.4 and 5.1°C, morning and afternoon RH 95 and 62 per cent, bright sunshine hours 4.9 and rainfall was 1.4 mm, all are conditions favourable for disease development. The R2 value of the regression analysis between observed and estimated SR prevalence was 0.98. Disease forecasting could provide the growers with information for well timed application of fungicides to control SR and this would be beneficial economically.

Evaluation of Brassica germplasm for field resistance against clubroot (Plasmodiophora brassicae Woron)

Of the 124 germplasm accession of oil seed Brassicas screened under field condition against clubroot disease (Plasmodiophora brassicae), 80% were susceptible and 17, 3, 1 and 1 of Brassica juncea, Brassica rapa var. toria, B.rapa var. yellow sarson and B. rapa, respectively, were resistant.

The Disease: Downy Mildew

The downy mildew disease is caused by the fungus Hyaloperonospora parasitica (Gaum.) Goker. The nomenclature of the disease is based on symptoms on the infected leaves with downy fungal growth. The upper surface of affected young and older leaves has ill-defined, irregular, pale yellow necrotic lesions, whereas the lower surface is covered by white-grey mycelium. The cotyledons and older leaves may be killed prematurely when single lesions coalesce to form large blotches. Diseased pods may be visible with angular brown lesions, and under high humidity, a sparse white-greyish mycelium with numerous sporangia and sporangiophores may be visible. Severe attacks may lead to premature ripening and drying of the siliquae. This pathogen, alone or in combination with Albugo candida (white rust), is responsible for causing severe losses in the yield of several temperate and tropical Brassicaceae crops, particularly rapeseed and mustard.

Association or Mixed Infection of Downy Mildew and White Rust Disease Complex

The association of downy mildew (DM) and white rust (WR) infection on oilseed Brassicas, vegetable Brassicas, wall flowers, and stocks has been observed. The association or mixed infections, or DM-WR disease complex, or simultaneous occurrence of A. candida and Hyaloperonospora parasitica on leaves, stems, inflorescence, and siliquae of crucifers in nature is very common. A. candida appears first on leaves, which probably predisposes the host to infection by H. parasitica. The intensity of mixed infections varies from 0.5% to 35%, depending on local weather conditions. Severity of mixed infections on leaves is not related to infections on inflorescence. It seems that greater susceptibility of new inflorescence and their availability over extended periods of time are associated with this phenomenon. The hypertrophied and malformed inflorescences of B. juncea infected with A. candida are usually covered heavily with the white powdery growth of conidia and conidiophores. Albugo candida alone, on artificial inoculation of flower buds, induces typical hypertrophy of the inflorescence.

Perpetuation and Survival of Pathogen

Oospores formed in malformed and senesced host tissues constitute an important means of survival of H. parasitica over periods under unfavourable environmental conditions. It is also known to survive through mycelium and conidia for a short period of time. Pathogen is also known to be active during oversummering and overwintering under unfavourable environmental conditions in the diseased debris. Germinating oospores, intact oospores, mycelia, and conidia of H. parasitica were observed in the severely infected macerated host tissues. The maximum conidial discharge of H. parasitica from kohlrabi leaves is between 5 and 6 a.m. (258 conidia/cm2). The conidial discharge decreases greatly from 12 noon to 8 p.m. Maximum conidial germination of H. parasitica from B. juncea was at 20 0C temperature. The germination of oospores of H. parasitica is dependent on temperature, light, pH of the medium, and age of oospores.

Infection and Pathogenesis

In crucifers, the downy mildew infection may be either general or local. In the former case, all or most of the leaves, and inflorescence, may bear conidiophores. Although some parts (especially the stem) may show no external injury, microscopic examination shows the mycelia are in the tissues. Generalized infection is restricted to young tissues, and seedlings may show completely infected leaves. Localized infection also occurs in young tissues, especially those still in active division. In the hypertrophy caused by Albugo, the cells of the epidermis and cortex are dividing and may readily give entrance to Hyaloperonospora. Young inflorescence may wholly or partly be infected, while normal tissues of older stems and leaves below the initial site of infection may remain free.