Fungi associated with esca disease in grapevine

Cross sections of woody stems of 309 diseased grapevines collected in France showed two kinds of necrosis typical of esca: a) A central light-colored necrosis of soft consistency, consisting of three zones, preceded by a centrally discolored wood, and b) a sectorial light-colored necrosis composed of two zones preceded by a sectorial brown necrosis. Isolations showed that different microflora was associated with each necrosis. Phaeoacremonium aleophilum and Phaeoacremonium chlamydosporum occurred in the discolored wood and the zones bordering the central decayed wood. Eutypa lata was the main fungus isolated from sectorial brown necrosis and the zones adjoining the decay wood. Phellinus punctatus was isolated from the sectorial and central decayed wood. Stereum hirsutum was present in decayed wood of 15 grapevines with esca symptoms not inhabited by P. punctatus. Wood decay tests and pathogenicity tests showed that S. hirsutum and P. punctatus were responsible for the decayed wood. Phaeoacremonium chlamydosporum and S. hirsutum produced a centrally discolored wood similar to that found in esca-affected vines. Phaeoacremonium aleophilum caused a sectorial brown necrosis of soft texture. From these studies, it was found that esca is a complex disease involving several microorganisms whose role in the process leading to wood degradation is discussed.     

Non-structural plum pox potyvirus proteins detected by immunogold labelling

Plum pox potyvirus (PPV) induces in infected Nicotiana clevelandii cells characteristic crystalline inclusions known as nuclear inclusions (NI) when located in the nucleus and as dense material (Dm) when located in the cytoplasm. Crystalline inclusions contain protease (NIa) and RNA-dependent RNA polymerase (NIb) proteins. It is now well established for all potyviruses that cylindrical inclusions contain CI helicase ATPase protein (Martin et al., 1992). The intracellular location of other non-structural PPV proteins remains unknown. Using Escherichia coli expression vectors, specific antibodies were obtained against P1, P3, 6K2 and NIb PPV proteins for which antibodies were not yet available. As expected, NIb antiserum labelled crystalline inclusions. P1, P3 and 6K2 proteins were present in both types of crystalline inclusions found in the nucleus and in the cytoplasm of PPV-infected leaves of N. clevelandii, suggesting that nuclear inclusions and dense material were composed of the same proteins. This composition is discussed.     

Control of the cowpea aphid, Aphis craccivora Koch (Homoptera: Aphididae), in cowpea, Vigna unguiculata (L.) Walp.

Cowpea, Vigna unguiculata (L.) Walpers, is a major dietary staple in tropical Africa where it is most often cultivated. The production is, however, greatly hampered by severe infestation and damage by insect pests including the cowpea aphid, Aphis craccivora Koch. The damage and bionomics of the aphid are briefly introduced. Chemical, cultural, physical and biological methods, and use of plant resistance for A. craccivora control in cowpea are reviewed. Finally, prospects for integrated pest management are discussed     

Comparison of an in vitro and a damping-off assay to test soils for suppressiveness to Pythium aphanidermatum

Testing of soil samples in greenhouse assays for suppressiveness to soilborne plant pathogens requires a considerable investment in time and effort as well as large numbers of soil samples. To make it possible to process large numbers of samples efficiently, we compared an in vitro growth assay with a damping-off assay using Pythium aphanidermatum as the test organism on tomato seedlings. The in vitro test compares the radial growth or relative growth of the fungus in soil to that in autoclaved soil and reflects suppressiveness of soils to the pathogen. We used soils from a field experiment that had been farmed either organically or conventionally and into which a cover crop (oats and vetch in mixture) had been incorporated 0, 10, 21, and 35 days previously. We obtained a significant, positive correlation between damping-off severities of tomato seedlings in damping-off assays and both relative and radial growth in vitro. In addition, radial and relative growth of P. aphanidermatum in the in vitro assay were positively correlated with several carbon and nitrogen variables measured for soil and incorporated debris. We did not find differences between the two farming systems for either growth measures of P. aphanidermatum or disease severities on tomato at different stages of cover crop decomposition. The in vitro assay shows potential for use with any fungus that exhibits rapid saprophytic growth, and is most suitable for routine application in suppressiveness testing.     

Frequency of transmission of artichoke Italian latent nepovirus by Longidorus fasciatus (Nematoda: Longidoridae) from artichoke fields in the Iria and Kandia areas of Argolis in northeast Peloponnesus, Greece

Artichoke Italian latent nepovirus (AILV) transmitted by Longidorus fasciatus is a causative agent of artichoke patchy chlorotic stunting (APCS) in northeast Peloponnesus in southern Greece. Populations of L. fasciatus collected from the Iria and Kandia areas of Argolis in northeast Peloponnesus were used in laboratory experiments to determine the frequency of transmission of the virus by its natural vector. One tenth to almost one half of the L. fasciatus specimens recovered from soil collected in two artichoke fields showing APCS transmitted AILV. Allowing nematodes access for 4 wk to Nicotiana clevelandii mechanically infected with AILV did not increase the number of individual specimens able to transmit virus. The total number of specimens transmitting virus in an experiment did not exceed fifty percent of the individuals tested. Virus-like particles were only observed adsorbed to the inner surface of the odontostyle and it is suggested that the high frequency of transmission of AILV by L. fasciatus is a result of efficient dissociation of virus particles from the specific sites of retention in the vector.     

Viruses in the northern potato-producing regions of Saudi Arabia

Agar double diffusion tests and, later, ELISAs were used to detect viruses associated with potato in 242 samples collected in 16 trips to Tabuk and Hail, northern regions of Saudi Arabia, in four consecutive growing seasons (autumn 1989, spring and autumn 1990 and spring 1991). Eleven different viruses were detected in Tabuk and 12 in Hail. The viruses detected in Tabuk were alfalfa mosaic (AMV), cucumber mosaic (CMV), tobacco mosaic (TMV), potato leaf roll (PLRV), tomato spotted wilt (TSWV), tobacco ringspot (TRSV) and potato A, M, S, X and Y. The same viruses, plus potato yellow dwarf (PYDV), were detected in Hail. AMV was most frequently and CMV least frequently detected in Tabuk, whereas in Hail the most and least common were PVA and PLRV respectively.     

Studies on the potential use of Pasteuria penetrans as a biocontrol agent of root-knot nematodes (Meloidogyne spp.)

Some aspects of the interaction of the bacterial parasite Pasteuria penetrans and the root-knot nematode ( Meloidogyne spp.) were investigated in laboratory and pot experiments. The variable spore attachment on juveniles exposed to water suspensions of the bacterium is probably attributed to differential susceptibility of biotypes within a heterogeneous Meloidogyne population. The relationship between spore concentration and attachment level is not linear over a range of spore dosages, indicating that even at very high spore concentrations the number of spores capable of attachment may not be present in excess and it is difficult to ensure sufficient numbers of spores to ensure infection will attach to all nematodes. Attempts to apply the bacterium in conditions such as might occur in seedbeds did not suppress nematode multiplication after transplanting in nematode-infested soil, indicating that the only effective application method is a thorough spore distribution in the planting sites. Two major constraints were revealed: high levels of spore attachment to juveniles does not always guarantee a significant reduction of egg laying and this is greatly influenced by the Meloidogyne biotype. Furthermore, the cumulative effect of the parasite in reducing Meloidogyne populations over several crop cycles was less than expected as the bacterium reduced intra-specific competition for the food supply and the less damaged root enabled many nematodes to survive.     

Preliminary investigation of the causal agent(s) of a disease causing leaf curl of tobacco in South Africa

A high incidence (86%) of potyvirus infection was noted in tobacco plants exhibiting a form of leaf curl in South Africa. Despite leaf curl being reported in the literature to be of geminiviral aetiology, no geminiviruses were detected. Furthermore, no other virus particles were detected by virus purification, TEM and serology. Twelve species of dsRNA were consistently isolated from these tobacco plants, but were absent from other forms of leaf curl-affected and healthy tobacco. Aphid and mechanical inoculation demonstrated that the purified potyvirus(es) did not cause leaf curl symptoms, but rather mild mottle and mosaic symptoms in tobacco. Partial characterization of the potyvirus preparation showed a possible relationship to a South African strain of potato virus Y. Because potyvirus-inoculated plants did not manifest leaf curl symptoms, and because leaf curl symptoms were noted in some plants not infected with a potyvirus, it was concluded that the potyvirus is not involved in the leaf curl aetiology, but causes a latent infection, the symptoms of which are masked. The pattern of the dsRNA banding, induction of enations and lack of mechanical and seed transmission are common to plant reoviruses. The possibility of a phytoreovirus involvement in this form of leaf curl is currently being investigated. The results from this study suggest that tobacco leaf curl disease worldwide, with regard to geminiviruses, be re-evaluated.