نشریه تحقیقات بیماری های گیاهی
سال سوم شماره 2 (زمستان 1393)

Late blight، caused by Phytophthora infestans، is the most important disease of potato in the world، which several models have been developed for its forecasting in different regions of the world. Two important and basic models are Hyre and Wallin’s which use weather variables BFD (favorable day based on daily temperature and rainfall) and SV (severity values based on hourly relative humidity and temperature) for disease prediction، respectively. In order to study of predicting potential of these models under the conditions at Gorgan region، weather data of ten years (2002-2012) were utilized via logistic regression and discriminant analyses. Analysis of these variables during two durations (from planting to disease appearance and from the first day of Farvardin to disease appearance) revealed that there isn’t a significant relationship between the models specified variables and disease occurrence، but changing the thresholds of precipitation in Hyre model and RH in Wallin model produced a significant relationship. Besides the variables of these models، the four variables of total precipitation، number of rainy days، total sunny hours and total evaporation، had a very significant correlation with disease occurrences، and could predict the disease occurrences with a high accuracy

This study was carried out to evaluate the effects of salicylic acid on the weight and number of potato mini-tubers and symptoms of Rhizoctonia solani in the potato mini-tubers (tuber sclerotia) and also on the total chlorophyll (a+b) and carotenoid contents in the leaves of potato plants under greenhouse conditions. The experiment was carried out in completely randomized design with factorial arrangement and 20 replicates. The potato mini-tubers (without any infection) were planted in pots containing pasteurized soil. The soil in half of the pots was inoculated with R. solani two weeks before planting. The 4 weeks-old plants (7-8 leaves) were treated by spraying the foliage with 0 (control)، 0. 2، 0. 5 and 1 mM salicylic acid weekly for 4 weeks. Results showed that concentration of 0. 5 mM salicylic acid reduced disease symptoms (number of sclerotia) on potato mini-tubers. Also، the results showed that application of salicylic acid increased the yield (number and weight of potato mini-tubers). Total chlorophyll (a + b) content in healthy potato plants was significantly higher than in potato plants infected with R. solani while amount of carotenoid content in healthy potato plants was significantly lower compared to infected plants. Also، results showed that chlorophyll and carotenoid contents increased in potato plants (healthy and infected) with increase in salicylic acid applied.

Phytophthora crown and root rot (Gummosis) is one of the most important diseases of pistachio trees in Iran for management of which various methods such as biocontrol have been recommended. Fluorescent Pseudomonads with the ability of producing a wide range of secondary metabolites such as different antibiotics and plant growth regulatory materials are among the most effective soil borne bacteria in biological control of plant diseases. Antagonistic fluorescent Pseudomonads were isolated from rhizosphere of infected (Gummosis) and healthy pistachio trees and their ability to control Phytophthora dreschleri was evaluated. A total of 75 Pseudomonas isolates emitting fluorescence in King B medium were obtained. In preliminary in vitro screening، seven isolates (R41، A16، A17، R14، A41، B21 and B13) showed inhibitory effect on mycelial growth of P. drechsleri based on bilateral culture test in PDA medium. The most inhibitory effect were obtained by R41 (≥15mm) and A16 (≥5mm) isolates which were selected for greenhouse tests on pistachio seedlings. The results of the greenhouse studies showed that R41 isolate exhibited greatest ability to reduce the disease severity.

Due to the adverse effects of agricultural pesticides on biological ecosystems، biological control of plant pathogens by using natural antimicrobial compounds such as essential oils and extracts of medicinal plants could be a necessity. Essential oil of carum copticum، is rich in antibacterial compounds particularly thymol. The aim of this study was to determine the minimum inhibitory and bactericidal concentration of Carum copticum essential oil against Pectobacterium carotovorum subsp. carotovorum as a plant pathogen and E. coli as a human pathogen in nutrient broth medium. In this study chemical compounds of Carum copticum essential oil were identified by GC-MS. Carum copticum essential oil at concentrations including 0. 004 to 4 % were prepared in nutrient broth medium. Antibacterial activity of essential oil in comparison with gentamicin antibiotic was determined by using 6 mm impregnated filter paper disks in agar diffusion method. The minimum inhibitory concentration (MIC) of Carum copticum essential oil against tested bacteria was determined by macrobroth dilution method. The MBC was determined by using cultivation method in nutrient agar medium. Zone of growth inhibition for Pectobacterium carotovorum varied from 5. 7± 0. 14 to 27. 4 ± 0. 95 mm while for E. coli it ranged from 6. 4 ± 0. 55 to 33. 1 ± 0. 95 mm in the order of increasing concentrations from 0. 004 to 4%. The MIC and MBC of Carum copticum essential oil against Pectobacterium carotovorum and E. coli were recorded as 0. 5% and 1% and 0. 125% and 0. 25%، respectively. Regarding the results of this research، plant essential oils can be a good alternative to chemical control of human and plant bacterial pathogens.

Using medicinal plants essential oils to control or suppress plant diseases has been considered by researchers all over the world. Fusarium oxysporum is one of the more common plant disease agents that affects a broad range of plants. In this study an in vitro experiment was set up to investigate the individual or combined effect of essential oils of rosemary and fennel on control of F. oxysporum. Treatments included five concentrations of essential oil of rosemary and fennel at 0، 25، 50، 75 and 100 ppm، using a completely randomized design (CRD) with five replications for each treatment. PDA plates with the desired concentrations of each essential oil were prepared and F. oxysporum was cultured on them. Results showed that essential oil of rosemary caused greater inhibition to fungal growth than that of fennel at all of the concentrations tested. Combined application of both essential oils (rosemary and fennel) had higher efficacy than individual application of them. Single application of essential oils of rosemary and fennel، at the highest concentration، inhibited fungal growth by 68/7 and 49/2% respectively، but combination of these essential oils completely restricted Fusarium growth (100%). Therefore، it is possible to restrict a pathogen more efficiently by application of lower concentrations of two or more essential oils in combination.

Fungal fusarium wilt and root knot nematodes are distributed in all tobacco-producing regions of the world and can result in yield losses in almost all tobacco-growing areas. Fungal soilborne pathogens can attack tobacco at any growth stage causing seedling death in both seedbeds and fields. Control of fungal soil-borne pathogens and root knot nematodes are accomplished through the use of pesticides، crop rotation and resistant cultivars. The use of resistant cultivars is preferable economic management because pesticides are expensive and may pollute the environment. According to these reasons this research was done with two aims; observation on the reaction of different flue-cured and air-dried tobacco varieties against fungal soil-borne pathogens and root knot-nematode، Meloidogyne incognita and identification of resistant cultivars to these pathogens. Therefore، this study evaluated reaction of 35 flue-cured and air-dried tobacco varieties to Fusarium oxysporum f. sp. nicotianae، and M. incognita in a factorial experiment with two factors including: tobacco type (flue-cured and air-dried tobacco varieties) and inoculum type (1-fungus alone، 2-Nematode alone and 3- fungus and Nematode) based on Completely Randomized Design with 6 replications in greenhouse at 25+3oC in Tirtash Research and Education Center in 2012. Each tobacco seedling was inoculated with 3000 eggs and second stage juveniles (J2) at transplanting into 1. 5 Kg pot and inoculated with the fungus 10-15 days later. After 70 days، number of egg masses، number of eggs per mass، reproductive factor، weight of leaves and roots were determined. The results showed a synergistic interaction between nematode and fungus on tobacco. Simultaneous inocoulation of nematode and fungus had greatly increased ratings of the symptoms and root necrosis severity. Results of variance analysis showed that there were significant differences among these cultivars and they had different reactions to fungal fusarium wilt and root knot nematode and that not all tobacco cultivars were resistant to pathogens. Based on ratings of the symptoms، varieties Bel 61-10، NC 100، Burley orumieh3 and HB4105P، showed resistance. Speight G-28، Ergo and Burley 21 were very susceptible.