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THE BLIGHT DISEASE

The Blight of potatoes, caused by the fungus Phytophthora infestans, is a disease extended all over the world that causes serious economical damages into the crop. Tubers are infected during the vegetative stage or during the  harvest, even if infection can also be transmitted through humid soils.

Spores over-winter on infected tubers; mycelial intercellular hyphae colonize the leaf tissues and causes cell death and forms specific lesions. In condition of high relative humidity white thick sporangiophores form around lesions. Zoospores proceding of infected leaf are carried by rain and wind, in this way fungus can be transmitted to potatoes foliage, tubers, etc. Disease symptoms appear between 3 to 7 days after contamination depending on the quantity of inoculum. In specific field conditions disease can extend rapidly and even cause complete loss of the crop in only 3 days.

Lesions on leaves and stems become visible as small flecks. Lesions expand to water soaked, gray green areas on the leaf.  They are often surrounded by a halo of lighter green tissue. The fungus may appear as a white, mildew-like growth at the edge of the lesion, primarily on the underside of the leaf. It is this white growth that distinguishes late blight from several other foliar diseases of potatoes. Tuber infections are characterized by patches of brown to purple discoloration on the potato skin. Cutting just below the skin reveals a dark, reddish-brown, dry, corky rot.

The fungus can complete many reproductive cycles in a season, accounting for the rapid increase of disease once it becomes established in a field. For this reason, an effective control of this disease requires implementing and integrated management approach. The most important measures had to be applied in a preventive way in order to reduce economical damages.

THE CONVENTIONAL CONTROL OF BLIGHT DISEASE

The control of the blight disease is becoming increasingly complicated due to the emergence of more virulent strains; these results from the genetic variability of this kind of fungi produced through their sexual reproduction.

All this has led to an excesive use of fungicides, that represents a great risk for the environmental because enhances contamination causing serios damages to human health. At the same time the excesive use of fungicides increases the production costs.

On the other hand the specific legislation concerning chemical product is incresingly restrictive, reducing the number of chemical poducts and maximum residue levels admitted. For this reason we are obligated to look for new strategies and alternative products respectful with the environmental.

ALTERNATIVE METHODS OF CONTROL

The use of the Glass line, based on salts of the phosphorous acid (H3PO3), represents a valid alternative to the application of conventional fungicides.  The tests carried out by José Morera, S.L in our laboratories have proved that only one application of Glass-K in potatoes plants, increase the content of specific flavonoides, the so called aurones, substances that belong to the group of the phytoalexins, related with the activation of the plant autodefence system.

FIELD TEST

During 2010-2011 different tests have been carried out in Mallorca (Spain), in 4 different plots of potatoes, variety Maris Peer; this genotype  is characterized by a good aptitude for the exportation (with good tuber characteristics), but with a great sensibility to the Blight disease. 

Three plots were planted in November and only one in January.

Products tested, were applied all together:

   - Glass-Mg, soluble liquid containing salts of Potassium phosphite and Magnesium phosphite. It was applied at a dose of: 0,3%.

   - Blaster, soluble liquid with micronutrients complexed with polyhydroxycarboxylic acids. It was applied at a dose of: 0,2%

   - BonActiv, biostimulator product based on amino acids and betaine. It was applied at the dose of: 1 L/Ha

The control plot was treated with conventional fungicides as Mancozeb WP a the dose of 4 to 7 Kg/Ha.

In both cases (treated and control plots) the number of treatement was the same, between 4 and 6. The first treatment was carried out between  4th y el 8th of February, in the plot planted in January the first treatment was carried out the 21th of March. Treatements were repeated every one or two weeks depending on the weather conditions. BonActiv only was applied in the 1st and 3rd  treatment.

RESULTS

Results showed a good effectiveness of products tested, in 2 plots no conventional treatment (with fungicide) was needed. In the other 2 plots only few symptoms of Blight were detected in both control plots and plots treated with our product (Glass-Mg, Blaster and BonActiv). In plots where blight was detected, Mancozeb and Cimoxanile treatments were carried out in a preventive way.

No difference was detected in kg of potatoes produced, but in plot treated with our product it was possible to observe a great tuber uniformity and consecuently a higher number of tuber comercially available.

In previous tests carried out by Jose Morera SL with potatoes var. Agria, it was possible to observe that a treatment with an higher dose of Glass Mg and Blaster (Glass-Mg a 5 L/Ha and Blaster a 3 L/Ha) increased the uniformity of tuber size, 90 % of potatoes presents a comercial size compared with the 80% of untreated plot. This treatment also increased the total production ammount of the crop, 7 Tm/Ha more than the untreated plot.

CONCLUSIONS

Observing these results we can confirm that Glass-Mg and Blaster represent a serious alternative to the conventional treatments in order to control  P. infestans in potatoes crops. At least they can be included in a crop defense strategy, alternatively with the conventional treatments. In this way is also possible to prevent the appearence of resistant strains of the fungus.

At the same time, Glass-Mg and Blaster are a very important nutritional source, complementary to the normal fertilizer support. For this reason this two products, together with BonActiv, can increase the quality  and the quantity of tubers produced in potatoes crop. 



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