Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/2/1997
Publication Date: N/A
Citation: Interpretive Summary: Postharvest losses can be as severe as 25% in developed countries and even greater in developing nations. Cantaloupe fruit has a short shelf-life due to field infections that pass undetected at harvest and manifest themselves during transport or at the market place by causing the fruit to rot. Information as to why these infections go dormant following initial infection and then become active following harvest is needed in order to enhance breeding for disease resistance and development of cultural control programs. Fungi degrade fruit with cell wall degrading enzymes. Their presence and activation in rotting tissues, and their enzymatic role in pathogenicity were investigated in tests conducted at Lane, OK. Fungal enzymes were identified which are involved in one of the most severe postharvest decays of cantaloupe grown in the southwestern United States. These enzymes were determined to be incapable of macerating immature fruit tissue, but caused extensive maceration in mature melon tissue. Preliminary indications are that there may be an inhibitor present in immature fruit that prevents the maceration. Manipulation of fruit defenses against these fungal enzymes will enhance fruit resistance and fruit quality.
Technical Abstract: Production of polygalacturonase (PG), a cell wall-degrading enzyme, by Phomopsis cucurbitae (latent infection fungus) was studied in relation to different carbon sources and various stages of cantaloupe fruit development. P. cucurbitae produced multiple PG isozymes both in vitro and in vivo. The fungus produced the highest PG activity and the greatest number of isozymes on pectin compared to those produced on glucose, galactose, and sucrose. Eight P. cucurbitae PG isozymes (pIs 3.7, 4.2, 6.6, 7.0, 7.3, 7.5, 7.8 and 8.6) were detected in extract from inoculated mature fruit (40-days after anthesis) by isoelectric focusing. Isozyme bands with pIs of 4.2, 7.3, and 7.8 were the most prominent. A similar set of PG isozymes was produced by P. cucurbitae in autoclaved mature fruit tissue (mesocarp). When fruit tissue discs, taken from 20-, 30-, 40-, and 50-day postanthesis fruit, were inoculated with P. cucurbitae, PG activity and the number of PG isozymes extracted from the macerated fruit tissue discs increased with degree of fruit maturity and ripening. Increases in PG activity and PG isozymes were also correlated with reactivation of latent infections and the beginning of tissue maceration. An anionic PG isozyme (pI 4.2) was only visualized on decayed 50-day old fruit mesocarp and 40- and 50-day old fruit mesocarp. The experimental results support the hypotheses that P. cucurbitae PG isozymes play an important role in fruit decay once latent infection becomes active following harvest.