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Arable Update November 2017 ArableADDING VALUE TO THE BUSINESSUpdate OF ARABLE FARMING Vegetable Seeds: Issue 4 White blister in radish seeds crop Introduction White blister (or white rust) is a disease caused by an oomycete pathogen - Albugo candida. This disease can cause severe damage to radish seed Key points crops and reduce yields. The pathogen is closely related to those which cause downy mildew diseases, Phytophthora spp. and Pythium spp. The • This Arable Update pathogen causes leaf, stem and seed pod spots, and more importantly, stag reports on three trials head symptoms resulting in poor flowering and economic yield loss (Rimmer, set up in 2016/17 to Shattuck and Buchwaldt 2007). better understand the development and spread Worldwide, Albugo candida has nine or more identified races which of white blister of radish are associated with various brassica species, including weeds such as seed crops. shepherd’s purse. Testing of races present in Western Australia has shown some cross-over between host species with the radish race also infecting • It was possible to detect Brassica juncea (brown mustard), which could therefore act as potential the presence of white inoculum source and aid in disease carryover between crops (Kaur et al. blister oospores in most 2011). They found that the race on shepherd’s purse was specific to that host. seed lines. However, to complicate the issue, the race associated with brown mustard • In a pot trial, disease could also infect oilseed rape, radish and shepherd’s purse. We don’t have development and severity good data on which Albugo candida races are active in New Zealand. did not appear to be influenced by oospore This Arable Update reports on three trials set up in 2016/17 to better loading on the seed understand the development and spread of white blister on radish seed crops. surface and all lines became infected. Experiment 1: Can spores be found in seedline? • White blister may infect the Sixteen lines of radish seed were evaluated for the presence of Albugo internal seed structures candida oospores by a washing and filtration method. On average, mean and act like an endophyte. oospore numbers varied between 0 and 1974 per 5g of radish seed. These Therefore the disease results identified lines of seed which provided a wide range of oospore is seedborne, infecting contamination for subsequent experimentation. the seedling plants when conditions are suitable. Table 1. White blister oospore counts per 5g of radish seed. • Ridomil Gold® MZ WG Seed-line Oospore Count Mean remains active against Replicate 1 Replicate 2 Oospores/5g white blister and forms 1 14 17 16 the base for an effective control programme, with 2 59 95 77 useful additions from 3 2826 1122 1974 Pristine®, Amistar® and 4 114 103 109 Seguris Flexi®. 5 12 5 9 6 38 58 48 • It is important not to rely 7 233 34 134 on one fungicide group 8 4 2 3 (e.g. Ridimol) as this 9 3 4 4 could increase the risk of 10 8 4 6 resistance developing. 11 13 4 9 • Increased white blister 12 12 2 7 disease control was 13 18 8 13 strongly related to 14 3 13 8 increased seed yield. 15 6 2 4 16 0 0 0 Experiment 2: Seed Transmission Methods Four radish seed-lines previously assessed for the presence of oospores of Albugo candida were selected for this experiment. Seed-lines chosen represented those evaluated with either not detectable (line 16), medium (lines 4 and 6) or high (line 3) levels of oospore contamination. Seedlings were established in planter bags in an attempt to prevent oospore contamination and limit the spread of the disease from one line to another. The experiment was replicated four times in a complete randomised block design. The trial was assessed for disease on 26 January 2017, 48 days after sowing. Individual plants were assessed for the number of leaves, total number of white blister lesions on the leaves, the total number of white blister lesions on the leaf petioles and an overall disease score (a scale of 0 to 5 where 0=no disease and 5=heavy disease). Results and discussion The number of lesions per plant varied greatly and ranged between 0 to more than 250. Even though there were large differences in detectable oospores between the four seed-lines tested, there were no differences between seed-lines in respect to white blister symptom development as measured by the total number of lesions per plant (P=0.34), mean number of lesions per plants (P=21) and mean disease score (P=0.31) (Table 2). This suggests that disease development and severity is not influenced by oospore loading on the seed surface and supports evidence in the literature that the pathogen infects the internal structures of the seed and may act as an endophyte within the plant. Table 2. Comparison of oospore seed contamination load (numbers per gram of seed) and subsequent mean leaf disease scores for four radish seed lines. Seed line Oospores / Mean disease gram of seed score (0-5) 3 395 0.83 4 22 1.70 6 48 1.63 16 0 1.17 Conclusion White blister infection is seedborne but was not related to surface spore loading. Experiment 3: Field evaluation of fungicides A trial was set up to investigate control options for white blister on hybrid red round radish located near Southbridge in the 2016/17 growing season. The trial was established early October and the grower managed all inputs except for fungicide application. Disease expression began later than in previous years but progressed steadily throughout the season (Figure 1). While the first infection was recorded in mid-December (first lesions on the untreated), all fungicides delayed the onset of visible symptoms until January. During February and March disease expression increased in all treatments to approximately 40% of racemes showing infection by the end of flowering (Table 3). Treatments containing Adexar® (trts 11 and 13) caused severe stunting of the crop and it did not recover prior to harvest, suppressing seed yield. Seed yield is presented for seed >2.6 mm and on a per hectare basis adjusted to include the area of male lines. Many fungicide products increased (P<0.001) seed yield above that of the untreated control. Repeated applications of Pristine® gave the highest seed yield while Ridomil Gold® MZ WG provided control levels similar to Amistar®, and an Amistar® plus Serguris Flexi® mix (Table 3). These products should form the basis of white blister control, however this trial tested products for efficacy and does not adhere to an anti-resistance strategy. Seed yield was highly correlated with the level of disease infection (Figure 2). White blister belongs to the Oomycete group of fungal diseases which contains pathogens known to develop resistance to multiple fungicide groups. Further work is required to investigate the optimum sequences and frequency for each family of chemistry to achieve effective control and manage resistance risk. For example Metalaxyl – M (e.g. Ridomil Gold® MZ WG) should only be used 2 – 4 times each season, while products containing both SDHI and Strobilurin fungicides should only be applied twice within the same season. Table 3. Fungicide treatment list applied to radish for the control of white blister disease near Southbridge in the 2016/17 growing season. Product Freq. #App Rate Application date l/ha 10.10.16 6.12.16 19.12.16 28.12.16 9.1.17 24.1.17 8.2.17 nil 0 0 0 - - - - - - - Rid 10 days 6 2.5 - yes yes yes yes yes yes Az (0.75) fb Rid 10 days 1 f.b. 6 2.5 + 10 Oct yes yes yes yes yes yes 0.75 Rid 10 days 6 2.5 - - - yes yes yes yes Pristine 10 days 1.5 - yes yes yes yes yes yes SF + Az 10 days 6 0.5 + 0.5 - yes yes yes yes yes yes Proline 10 days 6 0.5 - yes yes yes yes yes yes Rid f.b. Rid + SF 10 days 2 f.b. 2 2.5 + - yes yes yes yes yes yes f.b. Rid + Az f.b. 2 0.75 Foschek 10 days 6 0.5 - yes yes yes yes yes yes (phosphorus) Az f.b. Rid f.b. Az 10 days 1 f.b. 4 0.75 fb 10 Oct yes yes yes yes yes yes f.b 2 2.5 Sportac + 10 days 6 1.0 + - yes yes yes yes yes yes Adexar 1.25 Ranman (x3) fb 10 days 3 f.b. 1 0.2 - yes yes yes yes yes yes Pristine fb Ranman f.b. 2 Sportac + Adexar 10 days 6 (1.0 + 1.2) - yes yes yes yes yes yes f.b. Rid fb 2.5 Note: Az= azoxystrobin (Amistar); S.F. = Seguris Flexi; Rid = Ridomil Gold MZ WG; f.b = followed by. 60 50 y = 0.5675x - 24236 40 R2 = 0.92866 30 % Racemes infected 20 10 0 1.12.2016 16.12.2016 31.12.2016 15.01.2017 30.01.2017 12.02.2017 1.03.2017 Figure 1. Expression of White blister disease on untreated radish plots when grown near Southbridge during the 2016/17 growing season. Table 4. Seed yield (<2.6 mm) of hybrid radish following treatment with 13 fungicide combinations for the control of white blister, grown near Southbridge in the 2016/17 growing season. TRT# Product No. of Seed yield Disease (% racemes applications (kg/ha) with disease) 5 Pristine 6 1740 38 6 S.F +Az 6 1560 38 3 Az f.b. Rid + Az 1 f.b.
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