Ron Cohen

Toward Integrated Management of Monosporascus Wilt of Melons in Israel

The phenomenon of melon wilting dueto Monosporascus infection (Fig. 1) isknown in many regions around the worldand has been referred to as melon collapse(13,27,32,38), sudden wilt (2,6,8,26,29),root rot (17,39), vine decline (1,3,36), androot rot and vine decline (21–23,41,42).This disease is known also in the ’AravaRift Valley of southern Israel (8,19,26,31;Fig. 2), and the major causal agent isMonosporascus cannonballus Pollack &Uecker (Fig. 3). This pathogen is commonin hot, semiarid melon-growing areas ofIndia (22), southern Spain (13), southwest-ern regions of the United States (23,24,36),Saudi Arabia (15), Central America (1),Japan (40), Taiwan (37), and Tunisia (21).This disease in the ’Arava can be verysevere, capable of destroying the entirecrop (26), and will be referred to here asMonosporascus wilt. To date, disease man-agement in the ’Arava (38) has beenmainly based on methyl bromide fumiga-tion of the soil prior to planting. Sincemethyl bromide use will be prohibited inthe near future (33), there is an urgent needto develop alternative strategies for diseasemanagement.Melon root rot and vine decline causedby M. cannonballus has been reviewed in afeature article by Martyn and Miller (22),which describes the biology, pathology,and epidemiology of the disease, as well asmolecular methods for detecting variationin the pathogen population. In this article,we discuss approaches for the control ofMonosporascus wilt, with an emphasis onthe potential for integrated management, inview of the coming phaseout of methylbromide. These approaches include breed-ing for resistance, grafting melon plantsonto resistant Cucurbita and melon root-stocks, changes in irrigation schemes, im-proved soil solarization, chemical controlwith fungicides, and the use of other fumi-gants, alone or combined with soil solari-zation, to improve disease control.In field trials conducted by Reuveni andKrikun in the Jordan Valley and southern’Arava region in Israel (Fig. 2) in the early1980s, it was shown that Monosporascuseutypoides (apparently synonymous withM. cannonballus) is the primary agent ofmelon collapse (19,31). Pathogenicity testsperformed in 1995 and 1996 suggested thatthe most virulent species involved in themelon collapse syndrome in the ’Arava(Fig. 2) is M. cannonballus, although otherpathogens might also be involved (26).Monosporascus appears to be adapted tohot climates. This can be inferred from theclimatic conditions in the areas in whichthe fungus has been found and by itsgrowth temperature optimum. Vegetativemycelial growth is extensive in the rangeof 25 to 35°C, and perithecia formation invitro is optimal at 25 to 30°C (22).In commercial fields in Israel, the meloncrop can be totally destroyed by Monospo-rascus wilt in the autumn cropping season,whereas disease incidence and severity in acrop raised in the same plot during thefollowing winter–spring season can bemuch lower (2). Differences in soil tem-perature between crop seasons have beensuggested as a possible cause for such aphenomenon (17,28). This idea has beensupported by enhanced wilting obtainedfollowing artificial heating of the soil dur-ing the winter–spring crop season (28).Soil fumigation with methyl bromidebefore planting is the most common ap-proach for controlling Monosporascus wiltof melons in Israel. Methyl bromide haslethal exposure periods as short as 2 daysand can be applied at relatively low tem-peratures. The aeration period to eliminatevolatile residues before planting is short inmost soils, 3 to 10 days, allowing plantingshortly after treatment (18). In fact, meloncultivation in the ’Arava region is ex-tremely risky without methyl bromidefumigation prior to planting, due to theubiquity of M. cannonballus in ’Aravasoils that results in severe yield losses. Thephaseout of methyl bromide in developed

Variation in the response of melon genotypes to sudden wilt

SummaryDifferences in the response of melon genotypes to the sudden wilt disease were observed in several field trials conducted during 1993–1994 in the Arava region of southern Israel. Generally, the disease was more severe in the late summer growing season which is shorter and has higher temperatures than the spring and autumn growing seasons. The Oriental pickling melon breeding line P6a was the most tolerant among the entries tested. The response to the disease was also studied using two segregating families and their progenitors. BSK (tolerant) × P202 (susceptible) and P6a (tolerant) × D17 (susceptible). Wilting percentages of F1, F2 and backcross families were intermediate between the parents, suggesting an additive mode of gene action.

Single-gene resistance to powdery mildew in zucchini squash (Cucurbita pepo)

Cucurbita pepo (pumpkin, squash,gourd) is an economically important species that is susceptible to the cucurbit powdery mildew fungus, Podosphaera xanthii(syn. Sphaerotheca fuliginea). ‘True French’, an open-pollinated cultivar of the Zucchini Group of C. pepo, was crossed with an unnamed powdery-mildew resistant straight neck-type accession, the resistance of which was apparently derived from an interspecific cross with a resistant wild species of Cucurbita,and resistant plants were selected in the F2 generation. This was followed by six cycles of backcross-pedigree selection for resistance, and resulted in the development of an accession true-breeding for resistance to powdery mildew and nearly isogenic to ‘True French’. The resistant and susceptible near-isogenics were crossed and seeds of the filial and backcross generations were produced. Plants of the parental accessions and their progenies were grown together in a controlled-environment chamber, exposed to the pathogenic fungus, and scored as resistant, partially resistant, or susceptible 27–33 days after sowing. The results indicated that resistance is conferred by a single incompletely dominant gene, designated Pm-0.

Shading of plants facilitates selection for powdery mildew resistance in Squash

SummaryPowdery mildew development was assessed on squash (Cucurbita pepo) plants of a susceptible cultivar, a resistant accession, their F1, and their F2 in an early summer planting in the field, covered or not covered with a shading net. Three reaction types were observed: susceptible, powdery mildew on stems and on both, the upper and lower leaf surfaces, as in the susceptible parent; resistant, no powdery mildew on leaves or stems, as in the resistant parent; and partially resistant, powdery mildew on upper leaf surfaces only, as in the F1. Disease presence on the stem was associated with susceptibility. Shading hastened the appearance of powdery mildew and increased the severity of infection on partially resistant and susceptible plants, facilitating identification of resistant individuals in the F2 population.

Powdery mildew-resistant summer squash hybrids having higher yields than their susceptible, commercial counterparts

Attempts at breeding squash (Cucurbita pepo) resistant to powdery mildew (Sphaerotheca fuliginea and Erysiphe cichoracearum) have heretofore been unsuccessful due mainly to association of the resistance trait with reduction in yield. The goal of the present work was to determine if heterozygous hybrids expressing partial resistance could be potentially valuable for squash growers. One heterozygous hybrid each of zucchini, cocozelle, and vegetable-marrow squash was grown alongside a leading susceptible commercial hybrid cultivar of the same type, under standard field conditions. Although fungicides were applied at recommended rates every 6 to 13 days to suppress powdery mildew, the susceptible hybrids became heavily infested later in the season. The resistant hybrids were similar to their respective commercially available susceptible hybrids in fruit appearance and other horticulturally important traits. The resistant hybrids yielded at least as well as the commercial hybrids early in the season and outyielded them later in the season.

Trifluralin herbicide-induced resistance of melon to fusarium wilt involves expression of stress- and defence-related genes

SUMMARY To identify genes involved in trifluralin herbicide-induced resistance of melon to Fusarium oxysporum f. sp. melonis, suppression subtractive hybridization (SSH) and cDNA-amplified fragment-length polymorphism (cDNA-AFLP) were used. A total of 123 clones-60 of which have never been isolated from melon-were isolated, sequenced and annotated. A significant proportion (35%) of the total 123 clones exhibited similarity to genes that have been formerly described as stress- or defence-related. Thirty-two selected clones were subjected to a detailed expression analysis, one-third of which were found to be up-regulated in response to trifluralin treatment and/or fusarium inoculation. The putative roles of seven of these clones in stress are discussed. Furthermore, the expression of four stress-related and up-regulated genes was enhanced when the plants were subjected to salinity stress, suggesting that trifluralin induces a general stress response which protects the plant against fusarium wilt.

Origin and history of old cucurbit cultivars in Israel and the sources of several internationally important market types

Over the past century, Israel was a focal point for the improvement of local cucurbit landraces and introduction of cucurbit germplasm. Some improved open-pollinated cucurbit cultivars developed in Israel became established as market types of considerable economic importance far beyond its borders. The origin and history of these cultivars is not widely known and therefore the purpose of the present work was to collect and compare the records relevant to the development of these cultivars, and to describe them more fully. The four economically most important cultivars originated through mass-selection by amateur breeders and were named after their respective farming communities. The ‘Bet Alfa’ cucumber (Cucumis sativus) was selected from a local landrace and introduced in 1936. The ‘Malali’ watermelon (Citrullus lanatus) originated as a rogue in a local landrace and was commercialized around 1940. The ‘Ananas Yoqne‘am’ melon (Cucumis melo) was selected from a local landrace and commercialized around 1950. The ‘Ha‘Ogen’ melon was selected from a cultivar introduced from Hungary and commercialized in the 1950s. The outstanding fruit quality of these four cultivars resulted in their widespread planting in Israel and neighboring countries. Moreover, the quality of the cucumber and melon cultivars inspired successive improvements, notably introgression of disease resistance and development of hybrids, by Israeli breeder-geneticists. The ever-increasing demand for their high-quality fruits established as international market types the Bet Alfa cucumber, the Ananas Yoqne‘am melon, and the Ha‘Ogen melon and its derivative, the Galia melon, and each is intensively bred today by local and multinational seed companies.