Managing tomato vine decline with soil amendments and transplant treatments: fruit yield, quality, and plant-associated microbial communities.

Tomato vine decline (TVD) disease complex results in fruit yield loss, but what soil management strategies might mitigate it? In commercial fields with a history of TVD, five approaches (soil organic amendments and transplant treatments) were evaluated for their impact on fruit yield, fruit quality and microbial abundance or diversity at four site-years. One site-year had very high TVD pressure and high variability with no yield differences, thus efforts focused on the remaining site-years. Marketable yield was not different among treatments but numerically followed a trend similar to total yield. Amending soil with poultry manure delayed maturity (i.e., increased proportion of green fruit) and had the greatest total yield increases of 17.2%, congruent with decreased abundance of root pathogens (Verticillium dahliae, Rhizopicnis vagum). Microbial DNA fingerprinting data of rhizospheres, roots and/or stems suggested treatments did not significantly shift the total diversity fungal nor bacterial populations, but the aforementioned pathogen loads were reduced with the application of organic amendments relative to the untreated control. While drenching tomato transplants with pseudomonad culture increased their presence in roots, pathogen load was not reduced relative to the untreated control. Overall, these results show that soil organic amendments were able to improve tomato total yield in two of four site-years without reducing fruit quality (i.e., soluble solids, pH, colour), perhaps, in part, due to their ability to suppress specific root pathogens in commercial fields.

First Report of Macrophomina phaseolina, Fusarium brachygibbosum, and Lasiodiplodia theobromae Causing Fungal Watermelon Vine Decline in Southwest and West-Central Florida

Wilt and vine decline symptoms were observed on watermelon plants in Glades and Hardee Counties in Florida in spring 2017 that resembled viral watermelon vine decline caused by squash vein yellowing virus (SqVYV). When no SqVYV was detected, greenhouse studies and morphological and molecular analyses revealed three fungal pathogens, Macrophomina phaseolina, Fusarium brachygibbosum, and Lasiodiplodia theobromae, that were not previously reported on watermelon in Florida. A previously reported oomycete, Pythium spinosum, was also detected in some, but not all isolates, and but when applied independently, resulted in disease incidence that was comparable to the untreated check, ruling it out as a primary causal agent of the symptoms observed in the field. In one of three experiments, seedlings inoculated with a combination of Macrophomina phaseolina, Fusarium brachygibbosum, and Pythium spinosum suffered the highest disease severity based on AUDPC values. In another experiment, seedlings inoculated with F. brachygibbosum exhibited the most severe symptoms and rapid disease development following inoculation. When seeds were inoculated with either a single or a combination of the isolated fungi, those inoculated with L. theobromae resulted in seedlings with the greatest disease severity. This is the first report of these three fungal pathogens on watermelon in Florida.

Pathogenicity of Phytopythium chamaehyphon: a new player in Kiwifruit Vine Decline Syndrome of Actinidia chinensis var. deliciosa (A. Chev.) A. Chev. ‘Hayward’ in Italy

Kiwifruit Vine Decline Syndrome (KVDS) is a serious soil-borne disease that degrades the fine roots of both Actinidia chinensis var. deliciosa and var. chinensis. The disease seems to be the result of an interaction between several soil-borne pathogens, mostly oomycetes, and waterlogging. This work investigates the pathogenicity of the oomycete Phytopythium chamaehyphon recently isolated from roots of diseased plants. Pathogenicity was tested in 6-month-old and 1-year-old plants that after inoculation were flooded up to three times to induce symptom appearance. Leaf wilting and root rot typical of KVDS was observed in all the plants inoculated with P. chamaehyphon strain KD-15 (PCHA) and in all the positive controls potted in a mix of peat and soils collected in KVDS-affected orchards, while negative controls remained symptomless. Disease development on 6-months-old plants was characterized by unusual degradation of the not-lignified collar, occurring even in absence of flooding. Conversely, on 1-year-old plants, symptoms faithfully reproduced KVDS dynamics observed in orchard. This work confirmed the pathogenicity of P. chamaehyphon and raised new questions about the actual role of waterlogging in KVDS etiology.

GENETIC SIMILARITY OF Macrophomina pseudophaseolina ISOLATES ASSOCIATED WITH WEEDS IN THE BRAZILIAN SEMIARID REGION

ABSTRACT Macrophomina pseudophaseolina has recently been reported in association with weeds in melon producing areas in Northeastern Brazil. Species from this genus are the causal agents of root rot and vine decline (RRVD) in melon, reducing its productivity. It is needed to know the genetic variability of the pathogen to develop effective control methods. Thus, this work aimed to assess the genetic diversity among M. pseudophaseolina isolates collected from the weeds Trianthema portulacastrum L. and Boerhavia diffusa L. using ISSR and RAPD markers. For this, 41 M. pseudophaseolina isolates were submitted to amplification with five ISSR and ten RAPD primers. Genetic similarity was analyzed using the Jaccard’s coefficient and cluster analysis was performed by the UPGMA method. Combining data from both markers, the 41 isolates were separated into eight groups. Most groups were not arranged according to geographical origin and host of the pathogen. The genetic similarity among isolates ranged from 0.15 to 0.87. On the other hand, the highest genetic dissimilarity (85%) was observed between the isolate MpBr11, collected from T. portulacastrum in Icapuí (CE), and MpBr65, collected from B. diffusa in Assú (RN). Results obtained herein can assist breeding programs for the selection of resistance sources and the development of effective control methods against RRVD in melon.

Pathogens Associated to Kiwifruit Vine Decline in Italy

Kiwifruit production has gained great importance in Italy, becoming a strategic crop in several areas. In recent years, the Italian kiwifruit industry has been threatened by the emergence of a new, idiopathic syndrome causing a severe and rapid decline, leading to vine collapse within two years from symptom development. The main symptoms associated to this syndrome are the disappearance of feeding roots, blocking of both stele and xylem vessels, root cortex breakdown, leaf necrosis, phylloptosis, twig wilting and plant death. Kiwifruit decline affects both Actinidia chinensis var. chinensis and A. chinensis var. deliciosa. Due to the similarity with other fruit trees idiopathic diseases, such as the rapid apple decline, we propose to name this disorder as kiwifruit vine decline syndrome (KVDS). The causes of KVDS are still unknown. However, KVDS is prevalent in soils affected by waterlogging or poor aeration, suggesting a physiological origin of this disorder. In addition, our experiments suggested a role of the rhizosphere microbial community, since healthy and KVDS-affected plants show distinct bacterial and fungal communities. Phytophthora spp. and Phytopythium spp. were more frequent in symptomatic plants (58.6%) than in asymptomatic ones (19%). Moreover, Desarmillaria tabescens were found only on symptomatic plants. Inoculation of potted kiwifruit vines with those pathogens resulted in KVDS symptom development. Finally, induced waterlogging conditions increased the incidence of pathogen isolation, but not the symptom development.