Host Specificity and Genetic Diversity of Didymella bryoniae from Cucurbitaceae in Brazil

Abstract

Thirty-seven isolates of Didymella bryoniae from three Cucurbitaceae species were collected in Brazil and tested for pathogenicity to watermelon. All isolates were pathogenic but differed in aggressiveness levels. Seven representative isolates were used in cross-pathogenicity tests against 10 cucurbitaceous hosts. Most isolates were pathogenic to most host species tested, except to Sechium edule. Among the susceptible species, Citrullus and Cucumis species were the most susceptible hosts, while pumpkin and Luffa purgans were the most resistant. Host of origin affected the pattern of aggressiveness on each host. Isolates from watermelon were very aggressive to their original host, but much less aggressive or not pathogenic at all to some Cucurbita. Two previously described random-amplified polymorphic DNA (RAPD)-specific primers indicated that 81% of the isolates could be classified into the so-called RG I group, while the remaining isolates could not be classified into any of the described RG groups. All 37 isolates were further characterized by RAPD fingerprinting and compared with three US isolates representative of RG I and RG II groups. The Brazilian D. bryoniae isolates could be separated into genetically similar clusters. The majority of the isolates were grouped in cluster DB Ia, which contained only isolates of Citrullus lanatus and Cucumis melo. Two of the American isolates used as controls clustered with this group at 68% similarity level. The DB Ib cluster included three Brazilian isolates obtained from melon and watermelon and the American representative for RG II, at a lower similarity level (43%). Two isolates from watermelon clustered with one isolate from melon in a separate group (DB II), while one single isolate from pumpkin (DB III) showed the lowest genetic similarity to all other isolates. Didymella bryoniae isolates from Brazil showed, therefore, a level of genetic diversity higher than previously reported for the species. RAPD fingerprinting allowed for geographical distinction of D. bryoniae isolates but no correlation between genetic distance, aggressiveness or origin of the isolate was found.

Reaction of melon genotypes to Didymella bryoniae (Fuckel) Rehm

“…Some authors have reported the influence of environmental conditions and differences in the aggressiveness of isolates (Santos et al, 2009b;2013).…”

Control of gummy stem blight in watermelon through different management systems

“…In this period, the watermelon plants subjected to organic management showed higher AUDPC while the other treatments were statistically equal (Tukey test, P<0.05). The presence of rains, resulting in increased relative humidity, creates an ideal microclimate for emergence of fungal diseases, such as gummy stem blight (D. bryoniae) in watermelon and melon and black-sigatoka (Mycosphaerella fijiensis) in banana microclimate (UCHÔA et al, 2012;SANTOS et al, 2013). Moisture is essential for germination and germ tube penetration of most fungal spores by natural and/or injury of leaves and branches (SANTOS et al, 2013).…”

“…The cultivation of watermelon is easy to handle and with lower production costs compared to other vegetable crops. The incidence of diseases is considered one of the limiting factors to the production of watermelon, in view of the huge losses in productivity and fruit quality (SANTOS et al, 2013). Moreover, the incidence of diseases affects fruit appearance leading to its disposal and therefore increased cost of production (MIRANDA et al, 2005;LEÃO et al, 2008).…”

“…The fungus D. bryoniae can remain viable for up to three years in crop residues and weeds . The disease is manifested in the stem, leaves and fruits (SANTOS et al, 2009). In the leaves, one may notice circular patches of dark color, often with a yellow halo and with the disease progression there is aggravation of injuries that become brittle (MORAIS et al, 2008).…”

“…Although there are sources of incomplete resistance (SANTOS; , and fungicides are partially effective , disease progress is extremely fast, even from localized point-source inoculum (CAFÉ-FILHO et al, 2010). For control of gummy stem blight, the use of cultural practices such as rotation of non-host crops, fertilizer management, proper spacing, genetic and chemical control with the use of tolerant and/or resistant genotypes to disease are recommended (SANTOS et al, 2013).…”