GB2295772A - Fungicidal treatment of tubers - Google Patents

Abstract

Tubers of tuber producing plants are protected against infection by Phytophthora infestans by applying at least one phosphite compound to the plants by foliar spray.

Description

FUNGICIDAL TREATMENT The present invention relates to a method for treating tuber producing plants to protect tubers against attacks by fungal diseases.

It is known that a phosphite compound such as phosphorous acid and its derivatives (GB Patent No. 1,449,394) are active against some plant fungi namely Phytophthora species. However their action on Phytophthora infestans on leaves was found to be fairly poor which prevented any commercial application against this specific disease.

The fungus Phytophthora infestans is an important pathogen of potatoes attacking both the foliage and the tubers. Under commercial growing conditions the foliage first becomes infected and spores produced on these leaves are then washed down into the soil and infect the developing tubers.

In practice the disease is controlled by applying contact (copper, dithiocarbamates, chlorothalonil), penetrant (cymoxanil) or upwardly systemic (metalaxyl, oxadixyl, benalaxyl) fungicides to the foliage at regular intervals (7-10 days). These treatments protect the foliage and thus reduce the number of spores reaching the soil. In order to provide improved protection to the potato tuber, organotin compounds (triphenyltin hydroxide/acetate) are sometimes applied as cover sprays. These persist in the surface layers of the soil and kill the sporangia that fall from the foliage before they reach the tubers.

With current agricultural practices therefore potato tubers are protected from blight attacks by preventing the sporangia Phytophthora infestans from coming into contact with the tubers. Currently there are no commercial fungicides available that, when applied to potato foliage, will give direct protection to the developing potato tubers.

It has now been surprisingly found that it is possible to protect tubers of tuber producing plants and particularly potato tubers against Phytophthora infestans by foliar application of a phosphite compound. Hence the invention relates to a new use of a phosphite compound in a method for protecting tubers of tuber producing plants.

The present invention accordingly provides a method for protecting tubers of tuber producing plants against infection by Phytophthora infestans (e.g. late blight) which method comprises the application by foliar spray of at least one phosphite compound to the plants.

According to the present invention "phosphite compound" means phosphorous acid and alkyl esters thereof and salts of the acid and esters. Phosphorous acid, also known as phosphonic acid, is the product of formula H-P(O) (OH)2 (abbreviated H3PO3). Its salts are known as phosphites or phosphonates, and can be mono- or di-salts, preferably alkali metal or ammonium salts, in particular Na2HPO3, K2HPO3 or (NH4)2HPO3, alone or in mixtures. As derivatives mainly esters can be mentioned such as alkyl (C1-C4) esters and salts thereof with e.g. an alkali metal, alkaline earth metal or aluminium.

On a practical level, the effective amount of the phosphite active material which is applied is generally from 0.5 to 5 kg a.i./ha, and preferably 1 to 4 kg a.i./ha.

Preferred phosphite compounds are those whose solubility in water at 200C is greater than 0.1 g/l, more preferably greater than 0.5 g/l and more preferentially still greater than 50 g/l. Compounds of lower solubility can also be used but it is then necessary to use more complicated formulations, for example, wettable powders or aqueous suspensions. The use of active materials of sufficiently high solubility as defined above is advantageous in that it makes it possible to treat tuber producing plants by using simple solutions, which is extremely economic.

The phosphite compounds are generally applied in the form of compositions comprising an agriculturally acceptable diluent or carrier and/or an agriculturally acceptable surface active agent. Such compositions are known and have been described for the phosphite compounds used in the present invention.

The following Examples1 given without implied limitation, illustrate the invention and show how it can be used.

Example 1: The plot was organized as a randomised block design with 5 replicates. Each plot consisted of 4 rows with 10 plants per row. Potato plants (cv. "Up to date") were planted on 18th May. The crop was located in Northern Ireland.

Seven treatments were applied by foliar application at 10 day intervals between 22nd June and 20th August respectively with: 1) mancozeb formulated as Dithane DF at a rate of 1275 g a.i./ha.

2) phosphorous acid at a rate of 2 kg a.i./ha partially neutralised to pH 6.4 with potassium hydroxide solution.

The potato foliage was artificially inoculated with a spore suspension of Phytophthora infestans containing 105 sporangia/ml on 8th July.

The crop was defoliated on 28th August and the tubers were lifted and assessed for late blight infection at the beginning of October.

Under these conditions 15.2% of tubers from plants treated with mancozeb were infested by late blight compared with 1.4% of tubers from plants treated with potassium phosphite. Thus the latter treatment provided an excellent protection whereas the standard treatment with mancozeb was relatively ineffective.

Example 2: The plot was organized as a randomised block design with 5 replicates. Each plot consisted of 2 rows with 5 plants per row. Potato plants (cv. "Dundrod") were planted on 15th May.

The crop was located in Northern Ireland.

Only one treatment was applied as a foliar spray on 29th July with respectively: 1) mancozeb formulated as DITHANE DF at a rate of 1275 g a.i./ha.

2) chlorothalonil formulated as BRAVO 500 at a rate of 1250 g a.i./ha.

3) phosphorous acid at a rate of 2 kg a.i./ha partially neutralised to pH 6.4 with potassium hydroxide.

A series of plants were left untreated.

The potato tubers were harvested on 28th July and inoculated by spraying them with a spore suspension of Phytophthora infestans containing 105 sporangia/ml and 104 zoospores/ml. Ten tubers were inoculated per plot. The tubers were assessed for late blight infection on 26th August.

Under these conditions the percentage of infestation by late blight was respectively: - 68% for untreated tubers, - 56k for tubers treated with mancozeb, - 66% cf tubers treated with chlorothalonil, - but only 6% for tubers treated with sodium phosphite.

Thus the treatment according to the invention provided an excellent protection whereas the standard treatments were ineffective.

Example 3: The plot was organized as a randomised block design with 5 replicates. Each plot consisted of 1 row 5 meters long.

Potato plants (cv. "Bintje") were planted on 11th May. The crop was located in the Lyon area of France.

Foliar treatments were applied with: 1) mancozeb formulated as DITHANE M45 at a rate of 1600 g a.i./ha: five treatments were applied at 14 day intervals between 1st June and 26th July: 2) Fosetyl-Al formulated as ALIETTE 80 WP in two ways: a) a range of rates of Fosetyl-Al: 1600, 2400 and 3200 g a.i./ha as a single spray application on 10th August, 15 days prior to foliage destruction.

b) Fosetyl-Al, at a rate of 3200 g a.i./ha, six times at 10 to 14 day intervals over the period 1st June to 10th August.

A series of plants were untreated.

The tubers were harvested on 20th September and subsequently inoculated with a suspension containing 105 sporangia per ml of Phytophthora infestans.

The tubers were then assessed for late blight infection on 4th November.

Under these conditions whereas 80% of tubers of untreated plants were infected by late blight and 71k of tubers from plants treated with mancozeb were infected by late blight, only 33% of tubers from plants treated with Fosetyl-Al at 1600 g a.i./ha, 19 of tubers from plants treated with Fosetyl Al at 2400 g a.i./ha and 10k of tubers from plants treated with Fosetyl-Al at 3200 g a.i./ha were infected.

Thus the treatments according to the invention effectively protected potato tubers from infection by Phytophthora infestans whereas the mancozeb reference treatment was ineffective.

All these Examples clearly illustrate the remarkable protection of tubers against late blight achieved by the foliar application of phosphite compound to the potato plant.

Claims (8)

1. A method for protecting tubers of tuber producing plants against infection by Phytophthora infestans which method comprises the application by foliar spray of at least one phosphite compound to the plants.

2. A method according to claim 1, in which the phosphite compound is phosphorous acid or an alkali or alkaline earth metal salt thereof.

3. A method according to claim 1, in which the phosphite compound is an alkali, alkaline earth or aluminium salt of an alkyl (C1-C4) phosphonate.

4. A method according to any one of claims 1 to 3, in which the phosphite compound is fosetyl-Al.

5. A method according to any one of claims 1 to 4, in which the tuber producing plant is potato.

6. A method according to any one of claims 1 to 5, in which the phosphite compound is applied at a rate from 0.5 to 5 kg a.i./ha.

7. A method according to claim 6 in which the rate is from 1 to 4 kg a.i./ha.

8. A method according to claim 1 substantially as hereinbefore described in any one of Examples 1 to 3.