Postharvest application of organic and inorganic salts to control potato (Solanum tuberosum L.) storage soft rot: plant tissue-salt physicochemical interactions.

Soft rot caused by Pectobacterium sp. is a devastating disease affecting stored potato tubers, and there is a lack of effective means of controlling this disease. In this study, 21 organic and inorganic salts were tested for their ability to control soft rot in potato tubers. In the preventive treatment, significant control of soft rot was observed with AlCl3 (≥66%) and Na2S2O3 (≥57%) and to a lesser extent with Al lactate and Na benzoate (≥34%) and K sorbate and Na propionate (≥27%). However, only a moderate control was achieved by curative treatment with AlCl3 and Na2S2O3 (42%) and sodium benzoate (≥33%). Overall, the in vitro inhibitory activity of salts was attenuated in the presence of plant tissue (in vivo) to different degrees. The inhibitory action of the salts in the preventive treatment, whether effective or otherwise, showed an inverse linear relationship with water ionization capacity (pK') of the salt ions, whereas in the curative treatment, only the effective salts showed this inverse linear relationship. Salt-plant tissue interactions appear to play a central role in the attenuated inhibitory activity of salts in potato tuber through reduction in the availability of the inhibitory ions for salt-bacteria interactions. This study demonstrates that AlCl3, Na2S2O3, and Na benzoate have potential in controlling potato tuber soft rot and provides a general basis for understanding of specific salt-tissue interactions.

Ultrastructural alterations in Fusarium sambucinum and Heterobasidion annosum treated with aluminum chloride and sodium metabisulfite.

Aluminum chloride (AlCl(3)) and sodium metabisulfite (Na(2)S(2)O(5)) have received increasing attention as antifungal agents for the control of plant diseases. In an effort to understand their toxic action on fungi, ultrastructural changes and membrane damage in Fusarium sambucinum (Ascomycota) and Heterobasidion annosum (Basidiomycota) in response to salt exposure was investigated using transmission electron microscopy. Conidial membrane damage was quantified using SYTOX Green stain, which only enters altered membranes. The results showed that mortality of the conidia was generally closely associated with SYTOX stain absorption in F. sambucinum treated with Na(2)S(2)O(5) and in H. annosum treated with AlCl(3) or Na(2)S(2)O(5), suggesting that these salts cause membrane alterations. For both fungi, ultrastructural alterations in conidia treated with AlCl(3) and Na(2)S(2)O(5) included membrane retraction, undulation, and invagination. At higher concentrations or exposure periods to the salts, loss of membrane integrity, cytoplasmic leakage, and cell rupture were observed. Ultrastructural alterations and increased SYTOX stain absorption in salt-treated conidia appear consistent with a mode of action where AlCl(3) and Na(2)S(2)O(5) alter membrane integrity and permeability.

Effect of mycorrhization on the accumulation of rishitin and solavetivone in potato plantlets challenged with Rhizoctonia solani.

The effect of colonization with the vesicular-arbuscular mycorrhizal fungus Glomus etunicatum on the content of rishitin and solavetivone was determined in potato plants cv. Goldrush challenged with Rhizoctonia solani. Mycorrhization stimulated significantly the accumulation of both phytoalexins in roots of plantlets challenged with R. solani but did not influence phytoalexin levels in non-challenged plantlet roots. No accumulation of solavetivone or rishitin was detected in shoots. In Petri dish bioassays, rishitin and solavetivone inhibited mycelial growth of R. solani.

Effect of two vesicular-arbuscular mycorrhizal fungi on the growth of micropropagated potato plantlets and on the extent of disease caused by Rhizoctonia solani.

Two micropropagated potato cultivars, Goldrush and LP89221, were inoculated into sowing trays with either Glomus etunicatum or G. intraradices in a greenhouse. After 2 weeks, plantlets were transplanted into pots and roots were challenged 7 days later with Rhizoctonia solani. At different times after R. solani infection, disease severity, mortality rate, root colonization levels, various growth parameters, and shoot mineral content were evaluated. In Goldrush, only inoculation with G. etunicatum led to a significant reduction in disease severity, ranging between 60.2% and 71.2%, on both shoot and crown. This decrease was not observed in LP89221. Compared with the control plantlets, inoculation of Goldrush with G. etunicatum or G. intraradices reduced significantly the mortality rate by 77% and 26%, respectively, whereas vesicular-arbuscular mycorrhizal (VAM) fungi did not significantly influence the mortality rate in LP89221. In Goldrush, inoculation with G. etunicatum significantly increased shoot fresh weight, root dry weight and the number of tubers produced per plant, whereas G. intraradices only significantly increased the number of tubers. Tuber and root fresh weights of both potato cultivars were significantly reduced by R. solani infection. However, R. solani-infected plantlets of both Goldrush and LP89221, inoculated with G. etunicatum, produced significantly greater tuber fresh weight than non-VAM plantlets. In R. solani-infected plantlets of Goldrush but not LP89221, G. etunicatum and G. intraradices increased root fresh weight by approximately 140.3% and 76.5%, respectively, compared with non-VAM plants. The potato cultivars Goldrush and LP89221 responded differently to VAM fungal inoculation and to R. solani infection in terms of shoot mineral content.

Effect of Organic and Inorganic Salts on the Development of Helminthosporium solani, the Causal Agent of Potato Silver Scurf.

Potato silver scurf, caused by Helminthosporium solani, is an important postharvest disease of economic significance. Control of H. solani has been accomplished primarily by postharvest applications of thiabendazole. However, many strains of H. solani have become resistant to thiabendazole, resulting in failure of disease control. Consequently, alternative control strategies are needed. This study showed that several salts significantly reduced silver scurf development on potato tuber at a concentration of 0.2 M and that the timing of application also influenced salt efficacy. Among the 23 tested salts, aluminum chloride was the only one reducing silver scurf severity when applied either 2, 4, or 7 days after H. solani inoculation. Aluminum lactate, potassium sorbate, sodium carbonate, sodium metabisulfite, and trisodium phosphate also markedly reduced silver scurf severity but only when applied 2 or 4 days after inoculation. Ammonium acetate, calcium chloride, sodium benzoate, sodium citrate, and sodium formate reduced disease severity by at least 50% when applied 2 days after H. solani inoculation. With the exception of calcium chloride and sodium formate, these salts also were shown to strongly inhibit H. solani mycelial growth or spore germination in vitro. Results of this study further demonstrate the possibility of using selected salts for the control of potato silver scurf.

Selection of Antagonist Microorganisms Against Helminthosporium solani, Causal Agent of Potato Silver Scurf.

Silver scurf, caused by the fungus Helminthosporium solani, is an important disease affecting potato tubers. Control of the disease has been hampered by the development of H. solani strains resistant to thiabendazole, the only fungicide used in postharvest treatment. As a result, alternative control strategies are needed. In this study, 100 selected soil samples from the province of Québec were tested for their effect on silver scurf development on potato tubers. The results showed that 10 soils were able to decrease silver scurf development on tubers incubated at 10, 15, or 24°C. Many microorganisms were isolated from these soils and tested for their individual ability to reduce H. solani development using a whole-tuber assay. Several of them, including Alcaligenes piechaudii, Aquaspirillum autotrophicum, Arcanobacterium haemolyticum, Arthrobacter oxydans, Bacillus mycoides, Kocuria rosea, Streptomyces griseus, and a fungus of the class Zygomycetes displayed an ability to reduce the development of silver scurf on potato tubers at 10, 15, or 24°C. These results can find useful applications toward a biocontrol program of potato silver scurf as postharvest or seed tuber treatment.

Water activity control: a way to improve the efficiency of continuous lipase esterification.

During continuous lipase-catalyzed oleic acid esterification by ethanol in n-hexane, the oleic acid conversion, initially at 95%, decreases to 20% after 2 h. This decrease is caused by the accumulation of the water produced in the course of the reaction in the packed-bed reactor (PBR). In order to improve the PBR efficiency, it is necessary to evacuate the water produced. In this study, different approaches have been tested to control the water content in the PBR during continuous esterification. The first approach consisted in improving the water solubility by increasing the reaction medium polarity. The addition of polar additives to n-hexane, the use of more polar solvents, and the use of solvent-free reaction medium were tested as a means to favor the water evacuation from the PBR. First of all, the use ofn-hexane supplemented with acetone (3 M) or 2-methyl-2-propanol (1 M) enabled the conversion to be maintained at higher values than those obtained in pure n-hexane. The replacement of n-hexane by a more polar solvent, like the 5-methyl-2-hexanone, resulted in the same effect. In all cases, conversions at steady-state were always less than 95%, as obtained in pure n-hexane. This is explained by a decrease in the enzyme activity due to the increase in the medium polarity. Nevertheless, an increase in enzyme quantity allowed 90% conversion to be maintained during 1 week using 3 M acetone amended n-hexane. Good results (a steady-state conversion of about 80%) were obtained when esterification was carried out in a solvent-free reaction medium containing 2 M 2-methyl-2-propanol as a polar additive. The second approach consisted in the evaporation of the accumulated water by use of an intermittent airflow. Although this process did not enable constant esterification rate to be maintained, it did enable the initial conversion (95%) to be restored intermittently.

Production of Chitinases and beta-1,3-Glucanases by Stachybotrys elegans, a Mycoparasite of Rhizoctonia solani.

The in vitro production of chitinases and beta-1,3-glucanases by Stachybotrys elegans, a mycoparasite of Rhizoctonia solani, was examined under various culture conditions, such as carbon and nitrogen sources, pH, and incubation period. Production of both enzymes was influenced by the carbon source incorporated into the medium and was stimulated by acidic pH and NaNO(3). The activity of both enzymes was very low in culture filtrates from cells grown on glucose and sucrose compared with that detected on chitin (for chitinases) and cell wall fragments (for beta-1,3-glucanases). Protein electrophoresis revealed that, depending on the carbon source used, different isoforms of chitinases and beta-1,3-glucanases were detected. S. elegans culture filtrates, possessing beta-1,3-glucanase and chitinase activities, were capable of degrading R. solani mycelium.