Erwinia chrysanthemi in Israel - epidemiology and monitoring in seed tubers

In recent years, Erwinia chrysanthemi (Ech) on potato in Israel, in plants grown from imported Dutch seed tubers has been occurring more frequently causing economic damage. Disease symptoms first appear as wilt of the top leaves, which then spreads to the lower ones, followed by desiccation. Discoloration of the vascular system in the stem base is usually observed, followed by external darkening. In severe infections, the stem, and even the whole plant can dry out. Symptoms are usually associated with soft rot of the mother tuber, and sometimes (depending on level of infection) of the daughter tubers as well. The objectives of the present study were: a) to assess the impact of Ech-infected seed tubers imported from The Netherlands on disease expression in Israel, b) to develop a protocol for the detection of latent Ech infection in seed tubers. In spring 2004, disease was observed in several plots on imported cvs. Desirée and Mondial. In 2005, disease incidence on various imported cultivars ranged from 5 to 30% (8.2% on average) on more than 200 ha. In the autumn crop where domestic tubers harvested from an infested field were used disease incidence was 10-15%. In 2006, the disease was observed on more than 260 ha in various cultivars with disease incidence ranging from 2 to 30% (10% on average). Seed tubers sampled from commercial lots imported from Holland were tested for latent Ech infection, based on bio-PCR or enrichment ELISA. Out of 36 tested lots 24 were Ech-positive. Disease levels recorded in the fields in these lots ranged from 3% to 35% (10% on average), with only one exceptional case, where a low incidence of diseased plants was observed in a field originated from a Ech-negative seed lot. A protocol for detection of Ech in seed tubers was developed. A sample comprised of 200 tubers per 25 tons per lot divide into four or 10 replicates (50/20 tubers each), surfacesterilize with 0.5% NaOCl for 1 min, and the stolon end of each tuber was cut and placed in enrichment medium. After incubation of 48h, 0.2-ml aliquot was used for ELISA analysis or DNA extracted from the supernatant was used for PCR analysis. Our findings so far demonstrated the higher sensitivity of the bio PCR in comparison with the ELISA. This protocol is being tested also in spring 2007 with a large number of seed lots. Samples from these lots were planted in the field and Ech incidence is being recorded. Correlations between latent infections in the seed tubers and disease expression will be further studied, in order to evaluate the accuracy of the suggested protocol, and/or to improve it.vokMyynti MTT Tietopalvelut 31600 Jokioine

Environment-friendly agent [SG101] for the Control of Common Scab on Potato

Common scab on potato, caused by Streptomyces spp., can result in severe damage to pota-toes. The increased prevalence of this disease in Israel, especially in Ma'on region (South-West Israel), is attributable to several factors, among which are: susceptible crops such as peanuts and radish being grown in short rotations, soils that are conducive to the disease, and the absence of effective eradication methods. Contaminated potato seed tubers are the major means of spreading the pathogen. Seed borne inoculum arrives through certified seed lots imported from Northern-Europe for the spring. In average 26% of the seed lots were contaminated with intermediate and high levels of common scab between 2004 and 2007.However, so far any of the tested seed treatments was efficient in reducing the disease incidence or severity. The primary inoculum source of Sreptomyces is seed tubers. Once the pathogen is introduced and established into the soil, it survives for a long time, and only a radical soil fumigation (such as formaldehyde) can reduce the pathogen levels. The objective of the present study was to evaluate SG101, a stabilized formulation of hy-drogen peroxide agent (3.5-7% H2O2) as a seed treatment for controlling common scab on potatoes. Seed tubers (cv. Desiree) highly contaminated with common scab were treated with SG101 or Bactoril (Quat Ammonium) by a low volume spray, and mancozeb by dusting. Two experiments arranged in a randomized complete block design, with four replications were planted in two sites (Gilat -loess soil and Halutza - sandy soil). In Gilat, the incidence of progeny tubers with russet scab was significantly reduced in both SG101 and mancozeb treatments. In Halutza, the incidence of russet and common scab were significantly lowerin SG101, compared with the control and Bactoril. mancozeb treatment.The findings in this study indicate the high potential of using SG101 as a seed treatment for controlling scab in the short-term (in the harvested progeny). Furthermore, because of this SG101 efficiency, an advantage for the long-term is being achieved, by preventing the spread of the pathogen and infestation of soils. Additional studies on the potential use of SG101 as a furrow treatment and its effect on other seed-borne pathogens are being currently conducted.vokMyynti MTT Tietopalvelut 31600 Jokioine

Why have Colletotrichum and Helminthosporium become such a problem for fresh and ware potato quality - pathogen change or climate change

Both black dot and silver scurf caused by Colletotrichum coccodes and Helminthosporium solani, respectively, cause blemishes on the potato periderm. These diseases often occur together and frequently confused, cause reductions in fresh and ware potato production potential and damage value of seed potatoes. These diseases are largely distributed in all potato growing regions of the world, but black dot is most important in areas with dry and hot conditions, such as the Mediterranean regions, western USA, South Africa and southern Australia, although it is also well established in more temperate areas, such as UK, France, the Netherlands and Germany. Both diseases became a major problem in potato production over the past decade due to an increased demand for washed potatoes with a high-quality appearance. C. coccodes appears as a highly diverse species morphologically, physiologically and genetically; the existence of vegetative compatibility groups might explain this diversity, despite the fact that no sexual stage is known for the pathogen. C. coccodes is also diverse from a pathological point of view: it infects a range of plant species within and outside the Solanaceae, and shows some specific interactions with individual potato cultivars. In contrast, with H. solani there are no alternative hosts except to tuber-bearing Solanum species. Changes in cropping systems and in consumer dictate a reassessment of the economic importance of the skin blemish diseases, that severely affects the economic value of fresh market and seed potato production in regions of the world where external appearance is an essential driver of market access. An integrated disease management approach that includes cultural, biological and chemical control methods, in the field and in storage, is necessary for long-term control of skin blemish diseases. The fungicide TBZ was used for silver scurf control until the 1980 s, but due to high frequency of resistance imazalil has become the main fungicide used on seed tubers in Europe. However, C. coccodes is not sufficiently controlled by imazalil, suggesting another reason why black dot became a major pathogen on potato. Additional fungicides e.g. prochloraz, fludioxonil and azoxystrobin that control both pathogens are being introduced, however resistance to fludioxonil in H. solani was recently reported. Adequate control of the skin blemish diseases with very low tolerance for market access is highly problematic, and will require integrated strategies combining all currently available control methods.vokMyynti MTT Tietopalvelut 31600 Jokioine

Management of Verticillium Wilt of Avocado Using Tolerant Rootstocks

The global avocado industry is growing, and farmers are seeking to expand their plantations. However, many lands suitable for avocado planting were previously cultivated with hosts of the soil-borne fungal pathogen Verticillium dahliae, which is the causal agent of Verticillium wilt (VW). VW can seriously impair avocado orchards, and therefore, planting on infested soil is not recommended. The use of different rootstock types allows avocado cultivation in various regions with diverse biotic and abiotic constraints. Hence, we tested whether genetic variance among rootstocks may also be used to manage avocado VW. Six hundred trees, mostly Hass and some Ettinger, grafted on 23 selected rootstocks were evaluated for five years in a highly V. dahliae-inoculated plot for VW symptoms, fungal infection, and productivity. The selected rootstocks displayed a significant variation related to VW tolerance, and productive avocado rootstocks with potential VW tolerance were identified. Moreover, the rootstock productivity appears to correlate negatively to the susceptibility level. In conclusion, planting susceptible rootstocks (e.g., VC66, VC152, and VC26) in infested soil increases the likelihood of massive tree loss and low productivity. Whereas, tolerant rootstocks (e.g., VC804 and Dusa) may restrict VW and enable avocado cultivation on infested soils