Andreu1 A B, Guevara1 M G, Wolsky1 E A, Daleo1 G R and Caldiz D O2

Although strategies for controlling leaf and tuber diseases have been introduced over the years, serious losses still occur. As one alternative, chemicals that induce disease resistance have been tested. Our interest is focused on the molecular mechanism of induced systemic resistance (ISR), using potato – potato late blight as a model system, and on developing ISR technology for use as part of an integrated disease control strategy. In particular, the purpose of this work was to study the effect of β-aminobutiric acid (BABA) and fosetyl aluminium (Aliette) on late blight development, and on the production of resistance related molecules in potato varieties of industrial importance.

First, the resistance level of tubers and foliage of commercial cultivars to potato late blight, caused by Phytophthora infestans, was determined. Updated information on the late blight susceptibility of commercial cultivars is needed by crop managers and growers to optimize a successful use of integrated crop management strategies.

The commercial cultivars showed remarkable differences in their defense response to P. infestans isolate MC355 (a very aggressive Argentinean isolate) in laboratory, greenhouse, and field tests. Shepody was highly susceptible and Kennebec was moderately susceptible. Russet Burbank was moderately resistant. Santana and Russet Ranger were resistant and Innovator was highly resistant. Foliage of Russet Burbank and Santana was more susceptible than tubers from the same plants.

For chemical induction of resistance, foliage of potato cultivars was sprayed with BABA, Aliette or water (as a control). After three days the foliage was inoculated with P. infestans. The following parameters were evaluated: a) development of disease symptoms in foliage, b) development of disease in tubers, c) the protein level of two enzymes, a b-1,3-glucanase and an aspartil protease, in tubers and d) the phenol and phytoalexin content in tubers.

The two chemical compounds tested were unable to protect either the resistant Russet Ranger or the highly susceptible Shepody. The moderately susceptible Kennebec and the moderately resistant Russet Burbank were protected after these treatments. High levels of protection against P. infestans were observed in foliage during early stages of growth (plants 30 days old). The resistant Santana was protected during early stages of growth, during which it is moderately resistant. Subsequently, a decrease in late blight protection of foliage resulted in plants 45–50 days old and at the initiation of tuberization, which could be the result of the downward mobility of photosynthates to tubers.

An increase in resistance to late blight was detected in both recently harvested tubers and tubers stored at 8°C for three months of susceptible Shepody and moderately susceptible Kennebec, and to a lesser degree, of the moderately resistant Russet Burbank.

Immunological analysis of crude extracts from disks taken from tubers of plants treated with foliar applications of Aliette or BABA were performed using an antibody against a basic isoform of b-1,3-glucanase (36 Kda) purified from intercellular fluid of potato leaves infected with P. infestans and an antibody raised against an aspartil protease (AP) purified from intercellular fluid of potato tubers infected with P. infestans. Western blot tests showed that AP and b-1,3-glucanase were induced in the tuber disks from susceptible to moderately resistant cultivars in a similar induction pattern. When the intensity of the immunological reaction was estimated by densitometry, the levels of ß-1,3-glucanase and AP were 3-4 fold in highly susceptible Shepody for both enzymes, and 6 fold for ß-1,3-glucanase and 2 fold for AP in moderately susceptible Kennebec. No significant differences were observed in the resistant Santana, which is moderately resistant in early growth stages.

Slices from tubers of plants treated with foliar applications of Aliette or BABA were inoculated with P. infestans. Seven days after inoculation, the phytoalexin and phenol content was determined. A higher accumulation (5 fold) of these compounds was detected in the infected tuber slices of plants of Shepody and Kennebec treated with Aliette or BABA, compared to the control tubers from untreated plants. The diameter of growth of P. infestans on the tubers slices was one tenth that of the control.

Induced systemic resistance treatments may offer the possibility of controlling both foliage and tuber blight and could have a major impact in reducing the over-wintering survival of P.infestans in tubers. Chemical activation of disease resistance in plants represents an additional option for growers to protect their crops from losses due to plant diseases. It may be the best chemical control option for some pathogens, when genetic resistance is not available or is insufficient. A synthetic chemical resistance activator must meet the same stringent set of criteria as fungicides regarding environmental and toxicological safety and reliability under practical conditions, and it must be economically advantageous for agrochemical producers, farmers and suppliers. Integration of this novel tool of induced plant defense into existing crop management programs may be possible.

--------------------------------------------------------------------------------

  1 Instituto de Investigaciones Biológicas, Universidad Nacional de Mar del Plata, Mar de Plata, Argentina.

 2 McCain Argentina SA, Balcarce, Argentina