Damages caused by gum blight on cucurbits and management methods

Gummy blight (gummy rot or cucurbit gummosis) is a disease caused by the fungus Didymella bryoniae (without. Mycosphaerella melonis; anamorph: Ascochita cucumis). This pathogen can infect several species of cultivated and wild cucurbits. In cultivated species, the disease has been more important in melons (Cucumis melo), Valencian and Noble types (C. melo there. reticulatus e C. melo there. canatalupensis), into parthenocarpic cucumber (“Japanese cucumber”) (C. sativus) and in watermelon (Citrulus lanatus). Damage can reach 100% when control measures are not employed.  

    In order to efficiently adopt measures to control gum blight, it is necessary to know some characteristics of the pathogen and the epidemiology of the disease. D. bryoniae It can survive from one crop to another by colonizing cultural remains from the previous crop and also in the form of dormant mycelium. These two forms of survival tend to be a source of primary inoculum for the culture to be installed. Furthermore, D. bryoniae It is efficiently transmitted by seeds. In noble melon and parthenocarpic cucumber crops, seeds have been the main primary inoculum source, with an incidence of up to 54% and 52%, respectively, and transmission to seedlings of up to 52% and 25%, respectively.

The pathogen lodges internally, both in the shell, in the perisperm and in the seed embryo. This primary inoculum has been one of the factors that most contributes to gum blight epidemics in these two agrosystems. It is worth remembering that the vast majority of noble melon and parthenocarpic cucumber seeds are imported and that there has been no information on the packaging about the health quality of these seeds.

Although D. bryoniae can be transmitted efficiently via seeds, it is rare to observe the symptoms of the disease in plants during the seedling stages; that is, sick seedlings are rarely noticed in the cells of trays containing substrate (before transplanting). After transplantation, around 20 to 25 days, the first symptoms of gummy blight appear in the culture. Generally on the stem, in the region where the cotyledonous leaves are inserted, or where the first definitive leaf is inserted, small spots appear, light in color and with a corky appearance. These spots evolve into larger spots, with longitudinal and transverse development occurring on the stem and becoming blackish, with the presence of a large number of pycnidia and perithecia. In a following phase, the stem becomes girdled, with consequent wilting and death of the plant. Recently, Gasparotto et al (2011) showed that D. bryoniae presents latent infection in melon and cucumber seedlings, that is, the seedlings are infected, but do not show symptoms of the disease

    These characteristics lead to interesting sequences in the epidemiology of disease in field crops and the form of control that has been used by farmers. As the seedlings are asymptomatic, control measures have not been used both in relation to seed treatment and treatment of seedlings in trays.

The initial symptoms of the disease in the crop occur in the stem, in the region where the cotyledonous leaves and the first definitive leaf intersect. From there, the disease spreads to other regions of the stem of the same plant or other plants. One of the most efficient dissemination routes has been the farmers themselves, through cutting and pruning tools. When sprouting a sick plant (asymptomatic or symptomatic) and then sprouting another healthy one, pathogen propagules are transferred. Life et al (2012) showed that only with the disinfestation of pruning instruments can there be a reduction in the disease by up to 21,9% in melon crops. Another aspect refers to the fact that spraying with fungicides has often produced unsatisfactory results. This is because, in many cases, the foliage is protected but the infection foci are in lower segments of the stem.

It should be noted that the biggest problem with gummy blight in melon and cucumber crops is stem infections. Symptoms of the disease on foliage are not common. That is why it is important to protect, as a priority, the stems of plants. Another feature of D. bryoniae, important for disease control, discovered by Gasparotto (2010), is the occurrence of systemic infection in melon. Recent work has shown that this phenomenon also occurs commonly in cucumber.

   

Strategies to interrupt or reduce the disease cycle

 2 - Spraying seedlings with fungicides before transplanting is a strategy that should preferably be associated with seed treatment. Although seed treatment can result in a drastic reduction in transmission to seedlings, there is usually no eradication of the pathogen from infected seeds. Therefore, it is common that, even with seed treatment, infection still occurs in seedlings with inoculum originating from the seeds. please note that D. bryoniae, normally, causes latent infection in seedlings. Therefore, even with an asymptomatic appearance, seedlings can be a source of inoculum for disease epidemics in post-transplant culture. Spraying seedlings with fungicide is likely to contribute to a significant reduction in the intensity of the disease in the crop.

3 - Spraying the crop with post-seedling transplant fungicide is another important measure. Even with seed treatment and/or spraying of seedlings with fungicide, the presence of primary inoculum may still occur in the crop, the source of which was the seeds. Also, cultural remains from previous cucurbit crops colonized by the pathogen, dormant mycelium and spores originating from other cultivated cucurbits constitute a source of inoculum for the disease to establish itself in the post-seedling transplant culture. Control efficiency depends on the fungicide used, the interval between sprays and the optimization of spraying technology. Added to these factors are the favorability to gummy blight and the predisposition of the host: inoculum potential, climatic conditions and nutritional condition of the crop.

4 - Grafting with immune rootstock is another alternative. Grafting is a technique frequently used in parthenocarpic cucumber cultivation and is being introduced in noble melon cultivation. One of the most used rootstocks has been the Shelpper pumpkin hybrid, which is immune to D. bryoniae. Studies have shown that the use of grafting on various hybrids of noble melons using Shelper pumpkin as rootstock resulted in a reduction in the severity of gummy blight by up to 34,2%. Grafting is considered an alternative control measure and when used alone does not provide practical control, requiring additional measures.

5 - Balanced plant nutrition is also important. Plants that present stress, mainly due to nutritional balance, are much more predisposed to D. bryoniae, with the disease reaching greater severity. Work developed by Penharbel (2011) showed that in hydroponic and “mulching” cultivation there was a reduction in the severity of D. bryoniae of 85,9% and 12,1%, respectively, in relation to soil cultivation. This is also a measure that requires other complementary control actions.

6 - The use of less susceptible hybrids is also recommended. Although there are no immune hybrids or hybrids with a high level of resistance, there are significant differences in susceptibility to D. bryoniae between cucumber and melon hybrids grown in Brazil. The Prince Hakusho noble melon hybrid is significantly less susceptible to D. bryoniae, than the Bonus II and Sun Rise hybrids. Parthenocarpic cucumber hybrids are significantly less susceptible to the pathogen than noble melon hybrids.

Click here to read the full article in issue 78 of Cultivar Hortaliças e Frutas.