In some cases copper salts and antibiotics like streptomycin and kasugamycin are selectively used against bacterial plant diseases. All such agents are only preventives and should be used before the appearance of the bacterial infection.

None of the agents is full effective in dozes harmless to the environment. The agents usually act directly on the bacteria. Only the copper salts agents can, to some extent, induce plant immunity. The well-known substances for protecting plant against bacterial diseases have several shortages: they are not effective, they cannot be applied for a wide range of plants, are not safe for the environment, induce immunity of the pathogens and do not induce a natural immunity of the plants.

From the Polish Patent Application P. 324094 a bioagent is well-know for protecting plants against bacterial diseases in form of an aqueous suspension of microcrystalline chitosan (MCCh) containing of 0.001-10% of MCCh with average molecular weight not lower than 2000 Daltons, preferably 10000-30000 Daltons, water retention value not lower than 100%, deacetylation degree not lower than 65%, average particle size 1-100 pm characterized by active biological inhibition of the growth of bacterial plant pathogens resulting from the direct effect on the pathogens and induction of a natural plant immunity. Considering the suspension form, the practical use of the agent is limited and, besides, the particle size in the suspension influences negatively the protective efficiency of the agent.

In following publications"Experimental Mycology"vol. 3, p 285-287,1979, "Physiological and Molecular Plant Pathology", vol. 41, p. 33-52,1992, "Phytopathology", vol. 84 p. 312-320,1994,"Molecular Plant-Microbe Interactions", vol. 7 p. 531-533,1994,"Experimental Mycology", vol. 8 p. 276- 281,1984 and"Physiological Plant Pathology", vol. 84, p. l 19-123,1982, the use of initial chitosan is described as elicitor or protective agent against fungal diseases like Fusarium solani, Capsicum annum, Catharanthus roseus, Aphanomyces enteiches. The initial chitosan was applied in the form of aqueous solutions in acides. Well-known agents, based on chitosan, considering formulation, can exert phytotoxic actions in case of plants. They insufficiently affect fungal diseases in vivo. Because of their form they selectively affect pathogenic fungi and do not demonstrate a controlled effect on fungi in vivo.

The possibility of a simultaneous effect on various fungi strains is lacking.

From the monograph"Nauka o chorobach roslin" (Plant Diseases Science)- Panstwowe Wydawnictwo Rolnictwa i Lesnictwa (National Publication of Agriculture and Forestry) Warsaw 1987 and from following publications: "Fitopatologia" (Phytopathology)-Pahstwowe Wydawnictwo Rolnictwa i Lesnictwa, Warsaw 1967,"Ochrona roslin" (Plant Protection)-Panstwowe Wydawnictwo Rolnictwa i Lesnictwa, Warsaw 1978 and"Podstawy praktycznej ochrony roslin" (Fundaments of practical plant protection) Wydawnictwo Akademii Rolniczej (Publications of the Academy of Agriculture), Lublin 1980 the methods are well-known to control fungi plant diseases by means of synthetic fungicides. Synthetic agents for plant protection are toxic for the environment and people. Besides, they are not capable to simultaneously control several strains of fungi. The application of synthetic pesticides results in acquiring antipesticide immunity by the pathogones in short time. Biocides with effective wide action against fungal diseases are presently not known.

From the Polish Patent Application P. 324095 a bioagent is well-know for protecting plants against fungal diseases in the form of an aqueous suspension containing 0.001-10% of MCCh with average molecular weight not lower than 1000 Daltons, water retention value not lower than 100%, deacetylation degree not lower than 65%, average particle size in the suspension 1-100 m, demonstrating an ability to inhibit the growth of fungal pathogens occurring in the soil and/or on the plants by direct influencing the pathogenetic factor and inducing a natural immunity of the plants. Considering the suspension form, the practical use of the agent is limited and besides, the particle size in the suspension negatively influences the protective efficiency of the agent.

From the Polish Patent P. 269984 and P. 269985 a method is well-know for limiting the viral infection of plants. The method consists in applying chitosan on the plants before the viral infection caused by mechanical transfer of the viruses. The applied chitosan is either in solution in organic acids with the chitosan concentration being 0.005-0.1% and average molecular weight of chitosan 5000-2000000 Daltons or as a suspension of MCCh in water or organic solvents containing 0.005-0.1% of the polymer with water retention value 100-500%. The application of the chitosan solution in organic acids is not convenient, considering acidity and phytotoxic influence on the plants and the water-washing of the chitosan salts. The application of MCCh is limited, considering its suspension form and particle size.

According to this invention, the agent for protecting plants against diseases is a gel of chitosan with pH from 6.3 to 7.0 containing not less than 0.001 wt% of chitosan with average molecular weight not lower than 10000 Daltons, preferably 20000-500000 Daltons, and deacetylation degree not lower than 65%, preferably 70-95%, characterized by biological activity to inhibit the growth of plant bacterial, fungal and viral pathogens. The agent may additionally contain water-soluble polymers like carboxymethylcellulose or methylcellulose in the quantity of 0.001 to 10 wt% to chitosan. These polymers support in the application of the gel on the plants.

In the agent according to the invention, a chitosan gel is preferably applied which is a product of its neutralization of its aqueous solutions in organic and/or inorganic acids like lactic, citric, glutamin, acetic or hydrochloric acids. The agent, according to the invention, enables an ecological protection of plants against local and systemic viral infection and bacterial and fungal infections with a lasting effectiveness under changeable atmospheric conditions. The agent also protects the plants in industrial garden and field cultures. The agent allows to limit the viral infection even in those parts of the plants which were not directly covered with the agent. This is possible thanks to induction by the agent of a natural plant immunity against viral infection. An advantage of the agent is the very high immunity of the plants acquired at concentration as low as 0.001%.

The agent, according to the invention, is up the standards of an ideal bactericide and fungicide. It can be applied to a wide range of plants and is entirely safe for the environment, animals and people. The application of the agent in plant protection eliminates contaminations of the environment thanks to its natural origin and full biodegradability. The plants and fruits can be safely consumed few days after the treatment with the agent.

Another merit of the agent, according to the invention, is its higher biological activity in comparison to other well-known commercial agents including microcrystalline chitosan and solutions of the initial chitosan in aqueous acids.

The agent, according to the invention, can be used in plant protection against viral, bacterial and fungal diseases. The agent, according to the invention, is in more detail illustrated with the following examples not limiting the extent of the invention.

Example 1.

Bean plants, after 12 days from sowing, were treated with gel form of chitosan acetate with pH=6.76, containing adequately 0.01 wt% and 0.001 wt% of chitosan, with average molecular weight Mv=504 kD and deacetylation degree DD=80.5%. The tested plants were washed with water after 2 days and, after drying, inoculated by mechanical rubbing the alfalfa mosaic virus ALMV with concentration of 5 Hg/cm3. Average number of infected necrosis as well as the degree of virus inhibition were estimated after 3 days from inoculation, in comparison to the reference bean plants not subjected to chitosan treatment. The obtained results are presented in Table 1.

Table 1. Gel form concentration Average number of Inhibition , % necrosis at half of leave, % 0. 01 1. 1 99. 3 0.001 1. 0 99. 5 0 (control) 163.8- Example 2.

Bean plants were treated with gel form of chitosan acetate with pH=6.5, containing adequately 0.01 wt% and 0.001 wt% of chitosan, with Mv=70 kD and DD=73.4%. Inoculation of viruses was carried out as in Example 1. The obtained results are presented in Table 2.

Table 2. Gel form concentration Average number of Inhibition , % necrosis at half of leave, % 0. 01 1. 1 99. 3 0.001 2. 5 98. 5 0 (control) 163.8- Example 3.

Bean plants were treated with a gel form of chitosan acetate, containing adequately 0.01 wt% and 0.001 wt% of chitosan, with Mv=70 kD and DD=87.6%. Inoculation of viruses was carried out as in Example 1. The results of test are presented in Table 3.

Table 3. Gel form concentration Average number of Inhibition , % necrosis at half of leave, % 0.01 1. 0 99. 5 0.001 2.0 98.8 0 (control) 163.8 Example 4.

Bean plants were treated with a gel form of chitosan lactate with pH=6.75, containing adequately 0.01 wt% and 0.001 wt% of chitosan, with Mv=150 kD and DD=73.4%. Virus inoculation was carried out as in Example 1. The test results are presented in Table 4.

Table 4. Gel form concentration Average number number Inhibition ,%,%necrosis at half of leave ,% 97.90.013.5 0.001 4.0 97.5 0 (control) 163.8 Example 5.

Bean plants were treated with a gel form of chitosan lactate with pH=6.75, containing adequately 0.01 wt% and 0.001 wt% of chitosan, with Mv=70 kD and DD=87.6%. Virus inoculation was carried out as in Example 1. The results are shown in Table 5.

Table 5. Gel form concentration Average number of Inhibition , % necrosis at half of leave, % 0. 01 0. 1 99. 9 0.001 1. 0 99. 5 0 (control) 163.8- Example 6.

Bean plants were treated with a gel form of chitosan hydrochloride with pH=6.30 containing adequately 0.01 wt% and 0.001 wt% of chitosan with Mv=150 kD and DD=73.4%. Virus inoculation was carried out as in Example 1. The results of test are presented in Table 6.

Table 6. Gel form concentration Average number of Inhibition , % necrosis at half of leave, % 0. 01 3. 3 98. 0 0.001 3. 5 97. 9 0 (control) 163.8- Example 7.

Gel forms of chitosan acetate with properties presented in Table 7 were estimated on their antibacterial activity using Pseudomonas syringae p. v. tomato bacteria on the agar-agar plates. Gel forms of chitosan acetate with concentration of 0.1 wt% were introduced on the plates with incubation time 48 h at 37°C. The results of in vitro test are presented in Table 7.

Table 7. Gel form properties Inhibition Mv DD pH, % kD % 70 87. 6 6. 30 60 189 97. 0 6. 30 70 504 80. 5 6. 76 70 Example 8.

Gel forms of chitosan acetate with properties presented in Table 8 were tested in respect of their influence on the infection of tomato plants by Pseudomonas syringae p. v. tomato bacteria. Tomato plants were sprayed with 0.1 wt% of gel chitosan forms before bacteria infection. Inhibition of bacteria infection was estimated by comparing the amount of systemically infected plants, treated and untreated with gel forms of chitosan. The results are shown in Table 8.

Table 8. Gel form properties Inhibition Mv DD pH, % kD % 150 73. 4 6. 50 85 189 97. 0 6. 30 70 504 80. 0 6. 76 85 Example 9.

Gel forms of chitosan hydrochloride (gel 1) with pH=6.60, Mv=70 kD and DD=87.6%, chitosan acetate (gel 2) with pH=6.76, Mv=504 kD and DD=80.5% as well as chitosan acetate (gel 3) with pH=6.30, Mv=70 kD and DD=87.6% were used at concentration of 0.1 wt% for the treatment of soil planted with Difenbachia, infected by the Myrothecium roridum fungi strain.

The Bravo 500 SC fungicide with concentration of 0.2 wt% was also used as the control. Two variation of experiment were made: A-sprayed on a soil with chitosan gels 24 h before infection, and B-sprayed on a soil 24 h after infection.

The results of necrosis estimation (in mm) are shown in Table 9.

Table 9. Agent type A B Estimation, after days 5 7 5 7 Control 8. 8 12. 1 8. 8 12. 1 Bravo 500 S. C. 7.8 11. 6 3. 5 5. 1 Gel 1 8. 4 11. 9 2. 0 3. 5 Gel 2 7. 5 12. 0 1. 6 3. 2 Gel 3 7. 4 11. 2 1. 9 3. 6 Example 10.

Gel forms of chitosan as in Example 9 were used for protection against Sphaerotheca panhosa var. rosae on the rose plants of Mercedes strain cultivated in green house conditions. The gel forms of chitosan were used at concentration of 0.1 wt%. The Saprol fungicide agent at concentration of 0.15 wt% was also used as the control. The results of test after 2 chitosan treatments used at one week interval are presented in Table 10. A plant paralysis degree estimation: 0- no disease effect, 1-up to 1 % of plant surface covered by fungi, 2-1.1 to 5%, 3 -5.1 to 10%, 4-10.1 to 20%, 5-more than 20% was adopted.

Table 10. Agent type Paralysis degree Control 5. 0 Saprol 4. 0 Gel 1 3. 5 Gel2 3. 3 Gel 3 3. 3 Example 11.

Bean plants after 12 days from sowing were treated with gel form of chitosan lactate with pH=6.75 containing 0.1 wt. % of chitosan with Mv=504 kD and DD=80.5% as well as 0.005% of sodium carboxymethylcellulose. The plants were then infected by AIMV viruses as in Example 1. The results of test are presented in Table 11.

Table 11. Gel form concentration Average number of Inhibition , % necrosis at half of leave, % 0. 1 1. 0 99. 5 0 (control) 163.8