Watermelon vine decline (WVD) caused by squash vein yellowing virus (SqVYV) is a new and emerging disease that has caused devastating losses for watermelon producers in southwest and west–central Florida ( Huber, 2006 ; Roberts et al., 2005
Since the mid-1980s, a syndrome known as mature watermelon vine decline (MWVD) has had a serious effect on watermelon (Citrullus lanatus Thunb.) crops in Southern Indiana. As efforts to identify a pathogen responsible for MWVD have been unsuccessful, we have examined cultural conditions that might contribute to the syndrome. Field conditions were simulated in greenhouse pot trials to assess the impact of one or more factors on watermelon growth. Alone, low organic matter, soil acidity, black plastic mulch, and liming did not significantly affect root fresh weight; however, when these conditions were combined, root fresh weight was significantly reduced. Alanap-treated watermelons in combination with simulated cultural conditions resulted in further reduction of root fresh weight and had symptoms similar to MWVD. Watermelon plants grown in Alanap-treated, aged soil (from the previous year's experiments) under combined deficient cultural conditions demonstrated increased symptoms of MWVD and susceptibility to the pathogens Rhizoctonia and Pythium spp. Alanap, N-1-naphthylphthalamic acid (NPA), is a preemergent herbicide that functions as an inhibitor of auxin efflux and is widely used by watermelon farmers to impede obnoxious weeds. Metabolism of Alanap in planta involves aryl amidases (aminopeptidases) that also function in defense responses. We hypothesize that negative cultural practices are likely to inhibit defense responses and watermelon resistance to residual Alanap, leading to MWVD. We suggest that MWVD incidence is increased by certain common cultural conditions and that the incidence of MWVD can be reduced by altering these cultural practices.
Melon vine decline (MVD) results in severe economic losses in melon crops ( Cucumis melo L.) throughout the Mediterranean area and in the United States. MVD is a complex disease produced by many soilborne pathogens such as Monosporascus
We conducted a field screening of melon (Cucumis melo L.) cultigens to identify potential sources of host-plant resistance to Monosporascus cannonballus Pollack & Uecker. Seed were sown in Speedling trays with inoculated or noninoculated media. Plants were transplanted into a field known to be highly infested with M. cannonballus. Cultigens were arranged in a randomized complete block with three replications in each treatment (fumigated/noninoculated, nonfumigated/inoculated). Vine decline (1 = no symptoms to 5 = dead vines) was rated at 78 and 90 days post-transplanting. Disease symptoms were most severe and occurred earliest in the inoculated, nonfumigated plots. However, natural infection by M. cannonballus occurred in the fumigated plots, as all root samples collected contained perithecia. The majority of cultigens (85%) showed moderate to severe vine decline symptoms. Mean vine decline ratings of melon types from the spring test, ranked from most tolerant to least tolerant, were: Charentais = Ananas = Galia > Misc. = Honeydew = Mixed > US cantaloupe. A Fall test of the most tolerant cultigens showed similar results, in that Ananas and Charentais types had the highest tolerance; however, Galia types performed poorly, which may be due to the different environmental conditions, or differing fruit loads on the vines in the two tests. `Deltex', an Ananas hybrid, showed the highest tolerance based on vine decline ratings in the two tests.
Monosporascus root rot/vine decline (MRR/VD), caused by Monosporascus cannonballus, is a serious disease of the major melon production areas of Texas, California, and Arizona. We have previously identified differing levels of tolerance in melon germplasm based on vine disease symptoms. This study was conducted to evaluate the yield response of commercial and experimental cantaloupe and honeydew hybrids subjected to MRR/VD. Thirty-nine and six cantaloupe and honeydew hybrids, respectively, were transplanted into a field highly infested with M. cannonballus in March 1995 in a randomized, complete block with 4 replications. The field was highly infested with Monosporascus cannonballus. `Caravelle' (very susceptible) and `Deltex' (tolerant) were included as control entries. Fruit were harvested at maturity and sized. Any fruit that did not mature completely due to vine death were counted as culls (unmarketable). Marketable yield of the cantaloupe entries ranged from 26.74% to 67.35%. The most tolerant hybrids were `SR103654', `Don Carlos', `Explorer', and `Ovation'. Marketable yield of the honeydews ranged from 8.43% to 41.46%, with `Morning Ice' and `Creme de Menthe' showing the most tolerance. The best performing hybrids were evaluated again the Fall 1995 and Spring 1996 seasons. In general, genotypes which matured later, and had a more dispersed fruit set, were more tolerant to MRR/VD. This supports previous data showing that high physiological stress (heavy, concentrated fruit load) leads to more severe and rapid vine collapse.
Vine decline is limiting muskmelon production in many growing areas. Monosporascus cannonballus Pollack and Uecker and Acremonium cucurbitacearum Alfaro-García, W. Gams, and J. García-Jiménez are the main causal agents of this disease in Spain. The wild accession Pat81 (Cucumis melo subsp. agrestis Jeffrey) has shown to be highly resistant in naturally infested fields and after artificial inoculations. In three greenhouse experiments conducted over two seasons, the root structure of Pat81 was examined and compared to the highly susceptible commercial cultivar Amarillo Canario (AC). Pat81 produced a more vigorous, branched, and longer root system, conferring to this accession a higher capacity for the uptake of water and nutrients, even after inoculation using naturally infested soil. To determine the plasticity of the root systems, the effect of five different soil substrates on root growth was assayed. The root morphology was highly influenced by the soil substrate. Differences between genotypes appeared at 10 weeks after transplanting using sand as soil substrate. An organic substrate made up of well-decomposed peat and sand minimized the genotype × substrate interactions, and facilitated root analysis. This substrate allowed bringing the sampling date forward to flowering (at 7 weeks after transplanting). The maximum root length, the number and size of lateral roots (diameter 0.5-1 mm) and branching order, consistently differed between the two genotypes in most of the assayed substrates. These easily measurable root traits can be used as selection criteria in healthy soils to breed a larger root system more tolerant to stress. In addition, in inoculated soils the greater root absorbent area and the reduced lesion intensity of Pat81 could have applications to increase vine decline resistance of cultivated melons. By using segregant populations derived from the cross AC × Pat81, we are trying to modify the root structure of muskmelon in order to offer a genetic alternative to the expensive strategy of grafting muskmelon varieties onto rootstocks resistant to soil stresses.
We conducted a field screen of 130 melon cultigens to identify potential sources of host-plant resistance to Monosporascus cannonballus. Seed were sown in Speedling trays with inoculated or non-inoculated media. Plants were transplanted into a field known to be highly infested with Monosporascus cannonballus. Noninoculated plots were planted in rows that were fumigated with Telone II. Cultigens were arranged in a randomized complete block with three replications in each treatment (fumigated, nonfumigated). A disease symptom rating (1 = complete death to 5 = no symptoms) was taken at 78 and 90 days post-transplanting. Disease symptoms were most severe and occurred earliest in the inoculated, nonfumigated plots. Natural infection by Monosporascus occurred in the fumigated plots as over 95% of root samples collected contained perithecia. At the second rating date, 108 of the 130 cultigens tested were classified as moderately to highly susceptible (rating < 2.5). The four most resistant genotypes had a second rating equal or close to 4.0 (`Galia', `Deltex', `Rocky Sweet', and `Charlynne'). A group of 14 genotypes showed moderate resistance with a second rating of 3.0. Included in this group were `Morning Ice', `Doublon', `Israeli', `MR-1', `Santa Clause', and `Primo'. The physiological stress of a concentrated fruit set increases severity of vine decline symptoms.
The effect of zero, one, and two fruits per vine on plant growth and reaction to Monosporascus root rot/vine decline were investigated. In the first study, four cultivars with differing levels of tolerance were evaluated (`Primo', `Deltex', `Caravelle', `Magnum 45'). Vine decline ratings were taken weekly during the harvest period for 4 weeks. Treatments with no fruit showed delayed and less-severe vine decline symptoms. Temperature also effected vine decline symptom expression. In a Fall test, with lower temperatures during fruit maturity, symptoms were delayed in all treatments and often absent in treatments with no fruit load. Vine decline symptom expression is greatly effected by physiological (fruit load) and temperature stress. A subsequent study was conducted to more precisely quantify the effect of various fruit loads on shoot/root partitioning and vine decline symptoms. In addition to growth parameters root disease ratings were taken. `Caravelle', the most-susceptible genotype, was grown under differing fruit loads as mentioned above in Weslaco and Uvalde, Texas. As fruit load increased, root size decreased. Increased vine decline symptoms were observed under higher fruit loads. The implications on germplasm screening and breeding for resistance will be discussed.
here were identified during a breeding program aimed at introgressing the tolerance to melon vine decline (MVD) of ‘Pat 81’ ( C. melo ssp. agrestis var. conomon ) into the genetic background of ‘Piel de Sapo’ ( C. melo ssp. melo var. inodorus
A disease of muskmelon (Cucumis melo L.) characterized by a vine decline and a cortical root rot was first observed in the Lower Rio Grande Valley of Texas in 1986. In 1990, isolations from diseased plants collected from four commercial production fields yielded the fungus Monosporascus cannonballus. Pathogenicity tests with eight isolates confirmed Koch's postulates; however, there were differences in aggressiveness observed among isolates. M. cannonballus is an ascomycete fungus that typically produces only one (rarely two), round, jet-black ascospore per ascus. There is no known asexual stage. Temperature optimum of one isolate was 35 C. The optimum pH for growth was 6-7, but it grew well up to pH 9. M. cannonballus was first reported on muskmelon in 1970 from Arizona and recently was found in Japan under glass house culture. The presence of this fungus in Texas marks only the third report of this species worldwide, although a similar species (M. eutypoides) is the cause of a collapse of melon plants in Israel.