The Effect of Fruit Load on Transpiration Rate and Plant Collapse in Melon (Cucumis melo L.) Infected with Monosporascus cannonballus

S. Pivonia

Arava Research and Development, Sopir Center, Israel

J. Kigel and J. Katan

Faculty of Agriculture, Rehovot, , Israel

R. Cohen

ARO, Newe Ya'ar, Israel

Abstract. The expression of the vine decline disease in melon plants, caused by the fungus Monosporascus cannonballus, is closely related to fruit load and fruit maturity. The symptoms of the disease usually appear close to the time of fruit maturation. Plants wilt and die a few days after the appearance of the first symptoms that are expressed as turgor loss by the leaves. The fungus invades the root system and induces tyloses formation in the root xylem. Consequently, rates of water uptake and translocation are severely reduced and finally the plant dies, due to tissue desiccation. Melon plants inoculated with M. cannonballus, from which ripening fruit were removed, did not collapse and continued to grow and flower during the entire growth season. Even fruit removal from plants showing first wilt symptoms resulted in plant recovery. In order to study the quantitative effects of fruit load on leaf transpiration, a comparative study was performed using healthy plants with normal fruit load (four to six fruit), and plants without fruit. Transpiration was measured in fully expanded leaves, placed near the first fruit (node 6 from shoot base) or distanced from the fruit (nodes 10 to 13 from shoot apex ). Fruit were removed when first wilt symptoms were observed (i.e., at the stage of fruit netting, 2 to 3 weeks before maturity). Substantial reduction in leaf transpiration, that was similar in the leaves at the two nodal positions, was found after fruit removal. These results suggest that in diseased plants, the presence of maturing fruit delays stomatal closure, thus increasing water loss and risk of wilting.

 

Vine decline of melons Cucumis melo L., also known as melon collapse and sudden wilt, is a worldwide problem, mainly in arid and semiarid regions (Martyn and Miller, 1996). This disease is a major problem in the arid Arava region of southern Israel, which is the main out-of-season melon growing area for the local market and export. The vine decline disease causes a rapid wilt of plants bearing fruit and may cause total crop loss. In Israel, field trials and inoculation experiments conducted in the early 1980s (Reuveni et al., 1983) showed that Monosporascus eutypoides was a primary agent of melon collapse in the Jordan Valley. In pathogenicity tests performed in the Arava Valley in 1995 and 1996, Monosporascus cannonballus was confirmed to be the most virulent species involved in the vine decline of melons (Pivonia et al., 1997). The Monosporascus cannonballus related vine decline phenomena

have been reported also in the southern United States, Central America, Japan, Tunisia, Saudi Arabia and Spain (Martyn and Miller, 1996).

The relationship between fruit load and symptom severity was demonstrated. Disease symptoms were delayed or absent in plants if fruit were removed prior to ripening (Wolff, 1994). The purpose of this study was to test the effect of various fruit loads on the transpiration rate of diseased versus healthy melon plants.

Materials and methods

Effect of fruit load on disease severity. The effect of fruit load on disease severity was tested in a trial conducted in a field with a history of the disease at the Zohar experiment station in the fall cropping season of 1995. Fruit were removed 30 days after transplanting when the first wilt symptoms were

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observed. New fruit were removed once a week until the completion of the experiment. Disease development was evaluated 40, 46, 49, 53, and 61 days after planting. Comparison was made among plants with three final levels of fruit load: zero fruit, one fruit, and untreated plants with an average of 2.5 fruit per plant. Each treatment was applied to 50 plants, i.e., 10 plants on a 5 m row with five replications.

Effect of fruit load on transpiration rate in diseased and healthy melon plants. The effect of fruit load on transpiration rates was tested in a trial conducted in the Yair experiment station in Hazeva. Plants were transplanted in July 1997 in the second growing season in soil that was artificially inoculated with M. cannonballus and in methyl bromide treated soils. The plants were grown under shading net transmitting 75 % of natural sun light. The plants (10 plants for each soil treatment), were planted 100 cm apart and trained to a wire attached to a metal frame. Fruit from half of the plants (5 out of 10) in both soil treatments, were removed 58 days after transplanting, when the first wilt symptoms were observed in the infected plants. New fruit were removed twice a week until the completion of the experiment. Transpiration rates were measured using a steady state

porometer LI-1600 (Li-Cor, Lincoln, Neb.). Measurements were conducted in the morning between 8 to 10 am, from two leaves per plant. The first from fully expanded leaves, placed near the first fruit (node 6 from shoot base) and the second, distanced from the fruit (nodes 10 to 13 from shoot apex ). Data presented are an average of the two measurements. Comparison was made among plants with two levels of fruit load: zero fruit and four to six fruit on each plant.

Results

Effect of fruit load on disease severity. A mortality of 98% of plants grown in the infested soil with an average of 2.5 fruit per plant was observed 61 days after planting. Plants with fruit thinned to one or zero per plant exhibited wilt incidences of 75% and 12%, respectively (Figure 1). Monosporascus cannonballus was the fungus most frequently isolated from roots of wilted plants in this experiment.

Effect of fruit load on transpiration rate in diseased and healthy melon plants. No differences in

Figure 1. Effect of fruit removal on wilt incidence. Bars (for the same date), with the common letter are not significantly different (P = 0.05).

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transpiration rates of leaves were observed between infected and noninfected plants until about one week before the first wilt symptoms appeared (Figures 2 and 3). Fruit removal from noninfected plants resulted in substantial reduction in transpiration rate. In noninfected plants, transpiration rates (H2O) recorded 2 days after fruit removal were 2.7 compared to 1.1 mmol·m­2·s­1 in the plants without fruit and these differences remained relatively constant until the completion of the measurements, 68 days after planting (Figure 3). All infected plants baring fruit wilted until 63 days after planting. Fruit removal from infected plants showing first wilting symptoms, resulted in recovery of the plants (data not shown). Those plants survived until the completion of the experiment, 40 days after fruit removal.

Discussion

The expression of the vine decline disease in melon plants, caused by the fungus Monosporascus

cannonballus, is closely related to fruit load and fruit maturity (Wolff, 1996). The symptoms of the disease usually appear close to the time of fruit maturation. Plants wilt and die a few days after the appearance of the first symptoms that are expressed as turgor loss by the leaves. The fungus invades the root system and induces tyloses formation in the root xylem (Alcantara et al., 1995). Consequently, rates of water uptake and translocation are severely reduced and finally the plant dies, due to tissue desiccation.

Infected melon plants, from which ripening fruit were removed did not collapse and continued to grow and flower during the entire growth season. Even fruit removal from plants showing wilt symptoms resulted in plant recovery.

 

Figure 2. Transpiration rate of melon leaves from infected plants, with and without fruit. Bars indicate se values. Arrow indicates time of fruit removal.

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Figure 3. Transpiration rate of melon leaves from healthy (noninfected plants), with and without fruit. Bars indicate se values. Arrow indicates time of fruit removal.

The relationship between vegetative and reproductive growth is well documented in plants including cucurbits. Fruit development curtails vegetative growth and fruit removal enhances vegetative growth (Leonard, 1962; Zack and Loy, 1981). The results presented in our study also indicate that fruit removal from infected plants prevent wilting. Fruit removal from healthy plants causes the same response, i.e., reduction in transpiration rate. The involvement of maturing fruit in the wilting of melon plants infected with M. cannonballus can be interpreted as a result of the regulation of stomata function in the leaves by the fruit. In diseased plants, in which water uptake and translocation is reduced due to root decay and tyloses in the xylem (Alcantara et al., 1995), the risk of leaf desiccation is increased during the warmest hours of the day. Recurring cycles of turgor losses, result in sudden wilt and death of the plant. It is suggested that in this disease, wilting is related to leaf transpiration rate which is regulated by the presence or absence of fruit.

Literature cited

Alcantara, T.P., S.L. Rasmussen, D.H. Kim, N. Obeker, and M.E. Stanghellini. 1995. Field tolerance of melons to Monosporascus cannonballus. Phytopathology 86:1192 (abstr.).

Leonard, R.D. 1962. Inter-relation of vegetative and reproductive growth with special reference to indeterminate plants. Bot. Rev. 28:353­410.

Martyn, R.D. and M.E. Miller. 1996. Monosporascus root rot and vine decline, an emerging disease of melons worldwide. Plant Dis. 80:716­725.

Pivonia, S., R. Cohen, U. Kafkafi, I.S. Ben Ze'ev, and J. Katan. 1997. Sudden wilt of melons in southern Israel: Fungal agents and relationship with plant development. Plant Dis. 81:1264­1268.

Reuveni, R., J. Krikun, and U. Shani. 1983. The role of Monosporascus eutypoides in a collapse of melon plants in arid area of Israel. Phytopathology 73:1223­1226.

Wolff, D.W. 1994. Genotype, fruit load and temperature affect Monospotascus root rot/vine decline symptoms expression on melon (Cucumis melo L. ), p. 280­284 In: M.L. Gómez-Guillamón, C. Soria, J. Cuartero, J.A. Torés, and R. FernándezMuñoz (eds.). Cucurbits towards 2000. Proc. VI Eucarpia Meeting Cucurbit Genet. and Breeding, 28­30 May, Malaga, Spain.

Zack, C.D. and B. Loy. 1981. Effect of fruit development on vegetative growth of squash. Can. J. Plant. Sci. 61:673­676.

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