1. Powdery Mildew of Peas:
Pathogen (Casual agent):
The powdery mildew of peas caused by Erysiphe polygoni, an obligate fungal parasite belongs to class – Ascomycota’s.
The fungal mycelium form white superficial patches on the leaf surface widen sends haustoria into host epidermis and infect the crop.
The disease first appears at the time of flowering of the pea plants. The characteristic symptoms are the formation of white powdery or dusty patches on both sides of the leaves and also on the tendrils, stems and pods. In advanced stages, the entire plant surface may be covered with white powder like mass. The superficial mass consists of mycelium and conidia of the fungus. Later on black dots may appear late in the season when the crop reaches maturity. The pod and grain formations adversely affected.
Control of Disease:
Good cultivation practice:
(i) Certified reliant seed variety should be grown like Rachna, L-116, T, 10, T-56, P-185, P-388, P- 658b and P-6587
(II), Rally sowing around November give the crop varieties sufficient time for growth and reproduction before the disease sets in – otherwise an epidemic form appear in January – February,
(III) Irrigation at intervals check the spreading of disease,
(IV) Proper Sanitation: infected plant debris should be collected and destroyed which can reduce the primary infection of the crop.
Spray fungicides like SulfeX, ElosaI, Thiovit, Mildex, Milstem, Cosan, Morocide, and Karathane, Hexasul or any oilier wettable sulfur at the rate of 3g/lit of water. Give first spray after initial appearances of the disease and 2nd spray after 14 days. Repeat spraying only if needed.
2. Bacterial Blight or Bacterial Leaf Blight of Rice (Paddy):
Xanthomonas oryzae and Xanthomon as campestris pvoryzae (Ishiyama). Bacterial leaf blight is widespread in Asia. It has been reported from Sri Lanka, China, Kampuchea, India, Japan, Indonesia, Taiwan, Vietnam, Philippines and Thailand. It has not been reported from any rice growing areas outside Asia. In Japan, where it is known since 1884, it is called as ‘white withering disease’. The bacterial nature of the disease was established by Ishiyama in 1992.
In India, it was first recorded in 1951 in Khopoli area in Maharashtra, but at that time, the pathogen was not identified. Srinivasan et. al. (1959) reported the bacterial blight to be caused by astrain of Xanthomonas oryzae. Studies have shown the disease to be present in most of the rice growing states of India. Since the introduction of new, high-yielding but susceptible rice varieties over a large area in recent years, the disease has become one of the most serious problems of rice cultivation in India (Srivastava et. al., 1967).
Under Indian conditions, the disease appears in two phases, viz., wilt or ‘Kresek’ phase and leaf blight phase:
(i) Wilt or Kiesek Phase:
This is the most destructive phase of the disease which results from early systemic infection. The leaves roll completely, drop and turn yellow or grey and ultimately the tillers wither away. Affected shoot may be completely killed in severe attack.
(ii) Leaf Blight Phase:
The symptom is characterized by the appearance of straw-coloured stripes with wavy margins generally on both edges of the leaves. These stripes usually start from the tip and extend downwards. The leaves turn straw-yellow. Yellowish bacterial ooze appears on the surface which dries into beads-tike encrustations if there is no rain for a few days. This blight phase usually appears4 to 6 weeks after transplanting. The grains are partially filled or become chaffy in the diseased plants. The pathogen survives on the infected rice stubbles and seeds and also on some weeds.
Control of Disease:
(i) Use disease free seeds from reliable sources.
(ii) Drain the standing water in the field from time to time.
(iii) Give balanced dose of NPK fertilizers after soil testing. Use of heavy dose of nitrogen aggravates the disease.
(iv) Spray the crop with a mixture of copper oxychloride and streptocycline. Apply 7-5 g streptocycline or Agrimycin-100 and 500 g copper oxychloride (Fytolan/Blitox-50) in 500 litres of water. Start spraying after 30 days of transplanting and repeat after 15 days interval.
(v) Seed soaking for 12 hours and treating in hot water at 53°Cfor30 minutes will make the seed free from bacterium.
(vi) Grow resistant varieties like Ratna, Pusa-2-21, IR-20, Prasad, Govind, IR-24,Jaya,Vijay, Shakti, RP-4-14, RP-5-32, RP-31-49-2, MRI-550.
(vii) In endemic areas of bacterial blight complete transplanting by last week of June.
(viii) The run-off water from the affected field to the adjacent rice field should be avoided.
3. Papaya Mosaic:
Causal Organism: Papaya Mosaic Virus:
The typical symptom appears on the leaves. The leaves show profuse mottling and puckering, chlorotic and malformed appearance and sometimes modification into tendril-Lice structures. The leaves are reduced in size. The old leaves get defoliated leaving only a tuft of small ones at the top. The infected plants bear only a few fruit which remain abnormally small. They get deformed and may also show large mosaic patches, rings or blisters. The leaf petiole is reduced in length and the top leaves assume an upright position.
The virus is transmitted by a number of species of aphids such as Aphis gossypii, A. malvae, A. medicaginis, Myzus persicae. In addition to papaya the virus can infect a large number of common cultivated cucurbits.
Control of Disease:
(i) Roguing of diseased plants and their destruction is the most effective way to check the spread of the disease.
(ii) Spray insecticides like Nuvacron 40EC or ROGOR 30EC @ 1 ml/litre of water to keep the vector (insects) population in check.
(iii) For transplanting only healthy seedlings should be used.
Rice is one of the most important staple crops around the world. Unfortunately, grain yield has decreased significantly because of rice bacterial leaf blight, which is caused by the pathogen Xanthomonasoryzaepv. oryzae (Xoo), the most important and well-known bacterial disease of rice in rice-growing regions. Bacterial leaf blight can cause leaf wilting, affect photosynthesis, reduce 1000-grain weight, and generally result in yield losses by 20%–30% and even 100% under severe conditions [1,2,3,4,5]. Although bismerthiazole and streptomycin are the main tools for controlling rice bacterial leaf blight in China, Xoo has developed high resistance to both these bactericides [6,7]. Therefore, the search for new antibacterial agents remains a difficult task, and such agents are greatly needed in the field of agricultural bactericides.
Sulfone derivatives containing 1,3,4-oxadiazole moieties have a broad spectrum of bioactivities, such as antibacterial [8,9,10], antifungal [11,12], insecticidal , herbicidal , anticancer , and anti-HIV-1  properties. Over the past few years, studies on the synthesis and bioactivity of sulfone derivatives containing 1,3,4-oxadiazole moieties have attracted considerable attention. We previously demonstrated that such sulfone derivatives (Figure 1) display potent antibacterial activities. Specifically, 2-(methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole (CAS Registry Number: 142225-95-4) showed the best antibacterial activity against tobacco and tomato bacterial wilts caused by Ralstoniasolanacearum (R.solanacearum) with half-maximal effective concentration (EC50) values of 8.29 and 19.77 μg/mL, respectively . However, in our previous work, we only reported and discussed the compound’s activities in the control of R. solanacearum. The biological effects of these sulfone derivatives containing 1,3,4-oxadiazole moieties against rice bacterial leaf blight were not reported, and the underlying mechanism of these compounds on rice bacterial leaf blight remained unclear.
Figure 1. Compounds previously reported against tobacco and tomato bacterial wilts.
Figure 1. Compounds previously reported against tobacco and tomato bacterial wilts.
In this study, we found that 1,3,4-oxadiazole-containg sulfone derivatives, which demonstrate potent antibacterial activities against R. solanacearum, were highly effective against rice bacterial leaf blight in vitro and in vivo. Meanwhile, 2-(methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole could stimulate an increase in superoxide dismutase (SOD) and peroxidase (POD) activities in rice, causing a marked enhancement of plant resistance against rice bacterial leaf blight. It could also improve the chlorophyll content and restrain the increase in the malondialdehyde (MDA) content in rice to considerably reduce the amount of damage caused by Xoo. Moreover, 2-(methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole could obviously inhibit the production of extracellular polysaccharide (EPS) and reduce the gene expression levels of gumB, gumG, gumM, and xanA.
2. Results and Discussion
2.1. In Vitro Antibacterial Bioassay
As shown in Table 1, all the tested compounds demonstrated potent antibacterial activities against Xoo, with half-maximal effective concentration (EC50) values ranging from 9.89 μg/mL to 63.59 μg/mL, which were even better than those of bismerthiazole (92.61 μg/mL) and thiodiazole copper (121.82 μg/mL). The antibacterial tests showed that when small electron-with-drawing groups (e.g., -F) at the 4-position and a methyl substituted sulfonyl substituent were attached to the oxadiazole 2,5-positions, the corresponding compound 2-(methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole presented the best antibacterial activity compared to the rest of the test compounds. Meanwhile, the in vitro activity against Xoo of the compound 2-(methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole, with an EC50 value of 9.89 μg/mL, was better than the activity against tomato bacterial wilt (19.77 μg/mL) and slightly below the activity against tobacco bacterial wilt (8.29 μg/mL).
Table 1. Antibacterial activities against Xanthomonasoryzaepv. oryzaeof the title compounds.
|Compds.||Toxic Regression Equation||R||EC50 (μg/mL)|
|2-(Methylsulfonyl)-5-phenyl-1,3,4-oxadiazole||y = 2.16x + 2.18||0.98||20.07 ± 1.21|
|2-(Ethylsulfonyl)-5-phenyl-1,3,4-oxadiazole||y = 1.52x + 2.77||0.98||29.00 ± 1.25|
|2-(Methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole||y = 4.13x + 0.89||0.95||9.89 ± 1.52|
|2-(Ethylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole||y = 3.28x + 1.61||0.96||10.80 ± 1.43|
|2-(Methylsulfonyl)-5-(4-chlorophenyl)-1,3,4-oxadiazole||y = 1.72x + 2.64||0.99||23.21 ± 0.98|
|2-(Ethylsulfonyl)-5-(4-chlorophenyl)-1,3,4-oxadiazole||y = 1.60x + 2.25||0.99||52.61 ± 1.08|
|2-(Methylsulfonyl)-5-(2,4-dichlorophenyl)-1,3,4-oxadiazole||y = 1.04x + 3.21||0.99||52.14 ± 1.05|
|2-(Ethylsulfonyl)-5-(2,4-dichlorophenyl)-1,3,4-oxadiazole||y = 1.43x + 2.42||0.97||63.95 ± 1.05|
|Bismerthiazole||y = 1.50x + 2.05||0.98||92.61 ± 2.15|
|Thiodiazole copper||y = 1.54x + 1.79||0.98||121.82 ± 3.59|
2.2. In Vivo Antibacterial Bioassay
The results are listed in Table 2. At 15th day after spraying, 2-(methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole had potent curative activity of 38.17% against rice bacterial leaf blight at 200 μg/mL, which was better than those of bismerthiazole (30.21%) and thiodiazole copper (29.51%). At 28th day after spraying, the protective activity of 2-(methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole against rice bacterial leaf blight at 200 μg/mL was 41.82%, which was superior to those of bismerthiazole (37.51%) and thiodiazole copper (25.58%).
Table 2. Control efficiency of the testing compounds against rice bacterial leaf blight under greenhouse conditions at the concentration of 200 μg/mL.
|Compds.||15 Days after Spraying||28 Days after Spraying|
|Disease Index (%)||Curative Activity (%) c||Disease Index (%)||Protection Activity (%) c|
|2-(Methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole||57.63 ± 3.51||38.17 ± 2.15 A||54.79 ± 2.78||41.82 ± 2.45 A|
|Bismerthiazole a||65.05 ± 2.26||30.21 ± 3.43 B||58.85 ± 3.12||37.51 ± 2.54 C|
|Thiodiazole copper b||65.70 ± 2.73||29.51 ± 4.76 C||70.08 ± 3.67||25.58 ± 2.42 B|
|Untreated blank control||93.21 ± 1.79||0||94.17 ± 2.55||0|
2.3. Field Trial against Rice Bacterial Leaf Blight
The results are summarized in Table 3. At 15th day after the third spraying, the control efficiency of 2-(methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole at 150 g ai/ha against rice bacterial leaf blight was 81.34%, which was better than those of bismerthiazole (76.19%) and zhongshengmycin (71.87%).
Table 3. Field trials of the testing compounds against rice bacterial leaf blight.
|Compds.||Dosage (g ai/ha) d||15 Days after the Third Spraying|
|Disease Index (%)||Control Efficiency (%) e|
|2-(Methylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole a||150||2.03 ± 1.22||81.34 ± 2.76 A|
|Bismerthiazole b||375||2.59 ± 1.54||76.19 ± 3.54 C|
|Zhongshengmycin c||45||3.06 ± 1.86||71.87 ± 4.33 B|
|Untreated blank control||0||10.88 ± 2.32||0|
2.4. Determination of SOD and POD Activities
SOD, a key enzyme that resists biological oxidation in plant, catalyzes the reduction of superoxide anions (O2−) to hydrogen peroxide (H2O2). The diminished capacity for O2− removal causes a decreased ability of progeria cells to minimize oxidative damage may be a key factor in the disease. It plays a critical role in the defense of cells against the toxic effects of oxygen radicals .
POD constitutes a class of enzymes extensively distributed in plants and it has been shown that POD plays an active role in metabolism. An important function attributed to POD in plants concerns lignin synthesis. In many cases, particularly for plant-microbe interactions, this has been suggested as defense responses of plants to the stress .
As shown in Figure 2