Survival of Alternaria brassicae and Alternaria brassicicola on crop debris of oilseed rape and cabbage

Alternaria brassicae and A. brassicicola lesions present on infected leaves of oilseed rape and cabbage placed outdoors on soil produced viable spores for as long as leaf tissues remained intact. For oilseed rape this was up to 8 wk and for cabbage up to 12 wk. On leaves exposed in November and January spore concentrations decreased with time but on leaves exposed between April and June spore concentrations increased up to 9-fold in the first 4–6 wk and then declined. On stem sections of seed plants of oilseed rape and cabbage similarly placed on the soil, the fungi produced viable spores for up to 23 wk with spore concentrations increasing up to 11-fold in the first 6–8 wk after harvest. These results indicate that infected debris of brassica crops remaining on the ground after harvest may provide a source of dark leaf spot infection which may be implicated in the spread of the disease within and between crops.     

Induced resistance against Alternaria brassicae blight of mustard through plant extracts

An experiment was conducted to study the effect of plant extracts on soluble sugar, soluble phenol and defence-related enzymes response against Alternaria blight in mustard crop. The efficacy of six selected plant extracts (5 and 10%) used as foliar sprays at 60 and 70 days after sowing and mustard leaves was used for investigation. The results indicate that soluble phenol and sugar content in mustard leaves significantly increases in response to spraying of Azadirachta indica seed kernel, Calotropis procera and A. indica leaf extracts. The soluble protein, viz. peroxidase, polyphenol oxidase and phenylalanine ammonia lyase content, was higher in mustard leaves sprayed with C. procera leaves extract, A. indica seed kernel and Allium sativum bulb extract.     

Climatic factors influencing spore production in Alternaria brassicae and Alternaria brassicicola

Sporulation in A. brassicae and A. brassicicola on naturally-infected leaf discs of oilseed rape and cabbage required humidities equal to or higher than 91.5% and 87% r.h. respectively. The optimum temperatures for sporulation were 18–24°C for A. brassicae and 20–30°C for A. brassicicola at which temperatures both fungi produced spores in 12–14 h. Above 24°C sporulation in A. brassicae was inhibited. At sub-optimal temperatures sporulation times for A. brassicicola were significantly longer than for A. brassicae with the differences increasing with decrease in temperature. Interrupting a 16-h wet period at 20°C with a period of 2 h at 70% or 80% r.h. did not affect sporulation in either fungus but a dry interruption of 3–4 h inhibited sporulation in both. Exposure of both fungi to alternating wet (18 h at 100% r.h., 20°C) and dry periods (6 or 30 h at 5565% r.h., 20°C) did not affect the concentration of spores produced in each wet period. Sporulation times were not affected by either the host type of the age of the host tissue. White light (136 W/m²) inhibited sporulation in A. brassicae with the degree of inhibition increasing with increasing light intensity. The effect of light on sporulation in A. brassicicola was not tested.     

Effect of soil-applied NPK fertilizers on severity of black spot disease (Alternaria brassicae) and yield of oilseed rape

Effect of soil application of eight combinations of NPK fertilizers on the severity of black spot disease (BSD), caused by Alternaria brassicae (Sacc.) Berk., and yield of short duration oilseed rape (Brassica campestris L) were investigated under both pot and field conditions in 1987–88, 1988–89 and 1990–91. The severity of BSD was significantly greater (36–48%) on plants grown in ground treated with NP (N 90 kg ha^–1+P 40 kg ha^–1) applied as urea and single superphosphate respectively than on plants from the unfertilized control (NoPoKo) (o). However, the severity of BSD was significantly smaller (25–33%) when K (40 kg ha^–1) was applied as muriate of potash than in plants from control and NP treatments. The effect of NK (N 90 kg ha^–1+K 40 kg ha^–1) in decreasing the severity of BSD was increasingly more pronounced than the effects of PK (P 40 kg ha^–1+K 40 kg ha^–1), NP and K (40 kg ha^–1) applications. The decrease in the severity of BSD due to K was due to increased production in plants of phenolics which inhibited conidial germination and decreased sporulation of A. brassicae.The decrease in the severity of BSD due to NK application gave consistently increased seed yield 68% more than those of control and other treatments. The K-fertilized plants also showed increased resistance to lodging, increased 1000-seed weight and decreased seed infection. Seeds obtained from K-fertilized plants showed good seed germinability and vigorous seeding growth.     

Pollen selection for Alternaria resistance in oilseed brassicas: responses of pollen grains and leaves to a toxin of A. brassicae

Summary The effects of destruxin B, a host-specific toxin of Alternaria brassicae that causes black spot disease in oilseed brassicas, were studied on in vitro pollen germination and pollen-tube growth of Brassica campestris var brown sarson, B. juncea, B. napus cvs Westar and Cresor, B. nigra and Sinapis alba. Pollen grains of B. nigra, B. juncea and B. campestris were the most sensitive and those of S. alba the least sensitive to the toxin. Effects of the toxin were also studied on the leaves of these species, and the degree of sensitivity of leaves of different species was comparable to that of their pollen grains. The results on the responses of pollen grains as well as leaves to the toxin are in agreement with the degree of susceptibility/resistance of these species to A. brassicae reported in the literature, indicating that the genes imparting susceptibility/restistance are expressed in the pollen, a prerequisite for pollen selection. Results are also presented which show that the toxin fed to the cut end of isolated inflorescence axis is readily taken up by the developing pollen and results in the inhibition of germination of susceptible pollen. This technique offers a simple and effective method for application of selection pressure to eliminate pollen grains susceptible to the toxin from effecting fertilization.     

Evaluation of some fungicides for seed treatment and foliar application in management of damping-off of seedlings and blight of rapeseed caused by Alternaria brassicae

Eighteen fungicides were first evaluated for their effects on growth of Alternaria brassicae and for ascertaining their fungicidal and fungistatic natures in artificial cultures. The chemicals emerging fungicidal in action, were later evaluated for their efficacy as seed treatment and foliar application in the management of damping-off of seedlings and blight of rapeseed separately. Of 18 fungicides tested, six fungicides, viz., Dithane M-45, Dithane Z-78, Ziram, Difolatan-80, Blitox-50 and Benlate completely inhibited the growth of the pathogen and were fungicidal in action. Thiram and Brestan-60, which also caused total growth inhibition, were, however, fungistatic. Benlate (0.1 %) followed by Dithane M-45 was best seed-dressing fungicide for controlling damping-off of seedlings. Dithane M-45 (0.2%) followed by Dithane Z-78 as foliar spray was most effective for controlling the blight and increasing the yield in field trials.     

Epidemiology of grey mildew and Alternaria blight of cotton

Grey (Areolate) mildew (Ramularia areola) and Alternaria blight (Alternaria macrospora) are two important fungal foliar diseases affecting cotton in India. Both the diseases are polycyclic in nature. The primary inoculum for grey mildew is through conidia or ascospores from infected debris and/or perennial cottons and the secondary spread is through primarily infected leaves. Whereas for Alternaria blight the spread is initially from seed-borne inoculum (in Gossypium herbaceum and Gossypium arboreum cottons) and/or crop debris and the secondary spread is from sporulating lesions on older leaves. Both R. areola and A. macrospora require a temperature regime of 20–30?°C with prolonged high humidity (>80%) and frequent rains for infection and disease development. However, it has been observed that cool weather coupled with prolonged dewy periods in the absence of rains has also been found conducive for the development of both the diseases. So, suitable epidemiological tools and models are required to predict the disease development, spread and to design suitable management practices.     

Detection of Genetic Variation in Alternaria brassicae by RAPD Fingerprints

Genetic variation in fourteen isolates of Alternaria brassicae collected from different geographical regions of the world was determined by RAPD (random amplified polymorphic DNA) analysis. Twenty random primers were tried to amplify genomic DNA of A. brassicae. Based on the PCR (polymerase chain reaction) amplification of genomic DNA of A. brassicae with four oligonucleotide random primers, fingerprints were generated for each isolate and the amplifed products were compared. Using this technique, intra- and intercontinental genetic variation among isolates of A. brassicae could be distinguished.     

Survival of Alternaria cheiranthi in soil

Summary Experiments were conducted to study the survival of Alternaria cheiranthi in wet and dry soil. As free spores, survival was demonstrated only in air-dry soil. In wet soil, lysis of the germ tubes commenced after the second day. On wallflower seeds, the pathogen was viable until the seeds were decomposed. It was concluded that the fungus showed a low competitive saprophytic ability in the soil     

Development of an engineered bioluminescent reporter phage for detection of bacterial blight of crucifers

Bacterial blight, caused by the phytopathogen Pseudomonas cannabina pv. alisalensis, is an emerging disease afflicting important members of the Brassicaceae family. The disease is often misdiagnosed as pepper spot, a much less severe disease caused by the related pathogen Pseudomonas syringae pv. maculicola. We have developed a phage-based diagnostic that can both identify and detect the causative agent of bacterial blight and differentiate the two pathogens. A recombinant "light"-tagged reporter phage was generated by integrating bacterial luxAB genes encoding luciferase into the genome of P. cannabina pv. alisalensis phage PBSPCA1. The PBSPCA1::luxAB reporter phage is viable and stable and retains properties similar to those of the wild-type phage. PBSPCA1::luxAB rapidly and sensitively detects P. cannabina pv. alisalensis by conferring a bioluminescent signal response to cultured cells. Detection is dependent on cell viability. Other bacterial pathogens of Brassica species such as P. syringae pv. maculicola, Pseudomonas marginalis, Pectobacterium carotovorum, Xanthomonas campestris pv. campestris, and X. campestris pv. raphani either do not produce a response or produce significantly attenuated signals with the reporter phage. Importantly, the reporter phage detects P. cannabina pv. alisalensis on diseased plant specimens, indicating its potential for disease diagnosis.     

Chitinase-mediated inhibitory activity of Brassica transgenic on growth of Alternaria brassicae

Chitinase, capable of degrading the cell walls of invading phytopathogenic fungi, plays an important role in plant defense response, particularly when this enzyme is overexpressed through genetic engineering. In the present study, Brassica plant (Brassica juncea L.) was transformed with chitinase gene tagged with an overexpressing promoter 35 S CaMV. The putative transgenics were assayed for their inhibitory activity against Alternaria brassicae, the inducer of Alternaria leaf spot of Brassica both in vitro and under polyhouse conditions. In in vitro fungal growth inhibition assays, chitinase inhibited the fungal colony size by 12-56% over the non-trangenic control. The bioassay under artificial epiphytotic conditions revealed the delay in the onset of disease as well as reduced lesion number and size in 35S-chitinase Brassica as compared to the untransformed control plants.     

Ultrastructural Changes in Brassica Leaves Caused by Alternaria brassicae and Destruxin B

Alternaria blight in Brassica spp (caused by Alternaria brassicae) is characterized by dark brown to blackish necrotic lesions surrounded by chlorotic areas on the leaves. Similar chlorotic lesions were mimicked by a chlorotic toxin (destruxin B) purified from the culture filtrate. Ultrastructural studies were performed to study and compare the changes caused by A. brassicae inoculation in vivo and its host selective toxin under in vitro conditions. Electron microscopy of healthy, chlorotic and necrotic portions of B. campestris leaves naturally infected with A. brassicae revealed considerable differences at ultrastructural level. The necrotic lesions showed plasmolysis with total disruption of cell organelles. The chlorotic lesions had normal plasma membrane but swollen mitochondria with reduced number of cristae and vesiculation of the envelope. Chloroplasts showed degeneration of granal fretwork with an increase in the number of plastoglobuli. Chlorotic lesions due to foliar application of destruxin B induced indentical canges in leaves.     

In vitro and in vivo bio-assay of phytobiocidal effect of plant extracts on Alternaria solani causing agent of early blight disease in tomato

The efficacy of six locally available plants extract was evaluated for their phytobiocidal effect on Alternaria solani the causal agent of early blight (EB) disease of tomato and was compared with commercial fungicide mancozeb under in vitro and in vivo conditions. Under in vitro conditions, Eucalyptus globus and Datura alba were found to be ineffective, while the remaining plants extract (Allium sativum, Curcuma longa, Melia azedarach, Zingiber officinale) significantly reduced A. solani growth on PDA. Increasing concentrations of A. sativum (0–25%) were found negatively correlated with growth of A. solani on PDA. Among the tested plants, A. sativum extract at 20% concentration was found most effective against A. solani witnessed by both in vitro and in vivo experiments result. It reduced EB disease up to 75.11% over positive control. Similarly, among the different plants extract, maximum plant height (76.25 cm), fruit size (57.50 cm³) and yield (511.30 g) were observed in A. sativum sprayed treatments (20%), beside mancozeb and negative control. Upon phytochemical analysis of these extracts, flavonoids, alkaloids, saponins, tannins, terpenoids, glycosides and steroids were detected. Present study showed that 20% concentration of garlic extract has the potential to reduce EB disease severity, while having no noticeable phytotoxicity.     

蓝莓毛色二胞枝枯病的病原菌

近期在山东胶南的蓝莓种植区发现一种枝枯新病害,被侵染的枝条变色坏死,甚至引起全株干枯死亡。通过组织分离法和玻璃毛细管法获得纯化菌株HMQAU140073。通过形态学观察、rDNA-ITS和EF1-α序列分析及柯赫氏法则验证,确定该病原菌为假可可毛色二胞Lasiodiplodia pseudotheobromae。这是假可可毛色二胞所致蓝莓枝枯病的首次发现。     

蓝莓拟茎点枝枯病的病原

近期在山东省蓝莓种植区发现一种枝枯病害。为明确其致病菌,通过单孢分离方法和接种试验获得3个菌株。通过形态学特征观察和rDNA-ITS序列分析,确定该病原菌为乌饭树拟茎点霉Phomopsis vaccinii,即越橘间座壳Diaporthe vaccinii的无性阶段。这是乌饭树拟茎点霉所致蓝莓枝枯病在国内的首次发现。     

Effects of inoculum concentration, leaf age and wetness period on the development of dark leaf and pod spot (Alternaria brassicae) on oilseed rape (Brassica napus)

Experiments were done under controlled environment and glasshouse conditions to study the effects of inoculum concentration, leaf age and wetness period on the development of dark leaf and pod spot (Alternaria brussicae) on oilseed rape (Brassica napus). On leaves of potted oilseed rape plants (cv. Bienvenu) inoculated with A. brassicae conidial suspensions, the severity (number of lesions cm^-2) of dark leaf spot increased as inoculum concentration increased from 80 to 660 spores ml^-1and as leaf age increased from 4 to 14 days. On pods on detached racemes of spring oilseed rape (cv. Starlight), the incidence of dark pod spot (% of pods diseased) increased as inoculum concentration increased from 80 to 10⁴spores ml^-1. Increasing inoculum concentration above 10⁴spores ml^-1did not increase the incidence but did increase the severity of dark pod spot. A minimum wetness period of 4 h was needed for infection of oilseed rape leaves (cv. Envol) by A. brussicue at 18°C and disease severity increased with increasing wetness period up to 12 h. The length of dry interruptions after 3–8 h of initial wetness affected the severity of dark leaf spot. A second wetness period increased the severity of dark leaf spot if the dry interruption was ≤ 6 h and if the first wetness period was ≤ 8 h. The incubation period of A. brassicae decreased from 3.5 to 2.5 days as inoculum concentration increased from 80 to 660 spores ml^-on leaves (cv. Bienvenu) at 17–25°C and from 3.8 to 1.0 day as inoculum concentration increased from 80 to ≥2 ≥ 10³spores ml^-1on pods (cv. Starlight) at 18°C.