Storage variability among short-day onion cultivars under high temperature and high relative humidity, and its relationship with disease incidence and bulb characteristics

In the tropics, onion (Allium cepa L.) bulbs are usually stored in shelters under ambient conditions resulting in severe storage losses. This study was aimed at determining whether variation in bulb storability exists among short- day onion cultivars and whether the trait can be improved through conventional breeding. Twelve onion cultivars with different degrees of storability were selected from preliminary experiments. Bulbs of selected cultivars were grown and stored for 3 months under ambient conditions. Observations were made on disease incidence at harvest, percentage diseased bulbs, and storage disease incidence of bacterial soft rot (BR (Pseudomonas gladioli pv. alliicola Burkholder)), black mold (BM (Aspergillus niger Tiegh.)), and fusarium basal rot (Fusarium oxysporum Schlechtend.:Fr. f. sp. cepae) after 3 months of storage. Data on bulb characteristics such as bulb fresh weight (FW), dry matter (DM) content, total soluble solids (TSS), and pyruvic acid content were recorded at harvest. Mean storage losses of cultivars ranged from 21% to 99% over 3 years. Diseases were the major causes of storage losses, with BR and BM being the most predominant. Performance of most traits (including storage losses) was significantly influenced by year (Y), cultivar (G), and Y × G interaction. Heavy rainfall during bulb development in 1997 may have contributed to higher disease incidence at harvest, higher percentage of diseased bulbs during storage, and lower DM, and TSS of the cultivars. Cultivars with good storability, such as 'Red Pinoy' and 'Serrana', were less sensitive to stressful environments and high disease pressure. Incidence of storage diseases was significantly correlated with DM (r = -0.65 to -0.84) and TSS (r = -0.66 to -0.87), as well as incidence of BR (r = 0.57 to 0.94) in each year. Thus, they could be good indicators for evaluating storability. Cultivars with good storability tended to have small bulbs, as average bulb FW was positively correlated with incidence of storage diseases. Disease incidences on 'Red Pinoy' and 'Serrana', both in the field and in storage, were significantly lower than in the other cultivars, indicating they are tolerant to major storage diseases and that they could be used as donor parents for genetic improvement of onion storability. ambient conditions. A survey of specific problems dealing with onion production and storage in tropical and subtropical regions by Currah and Proctor (1990), indicated that onion storage losses were serious in 44 out of 72 tropical countries. Survey respon- dents in most of the Asian countries (19 out of 21) reported high onion storage losses. Diseases were the primary problem, with sprouting being the secondary factor that limited storage under tropical conditions (Currah and Proctor, 1990). Onion storability is affected by environmental, pathogenic, and genetic factors. Hot and humid conditions promote develop- ment of storage diseases such as black mold (BM (Aspergillus niger)) (Tanaka, 1991; Yoo et al., 1989), fusarium basal rot (FR (Fusarium oxysporum: Fr. f. sp. cepae)) (Lorbeer and Stone, 1965), and bacterial soft rot (BR (Pseudomonas gladioli pv. alliicola)) (Wall and Corgan, 1994; Wright, 1993; Vitanov, 1976).