Development and Characterization of Wheat Germplasm with Combined Resistance to Russian Wheat Aphid and Stem Rust (Race “Ug99”) in Kenya

Wheat is the second most important cereal in Kenya. However, production is severely constrained by both abiotic and biotic stresses. Of the biotic stresses a devastating pest (Russian wheat aphid (RWA)) and a serious disease (stem rust race TTKS (“Ug99”)) are currently the biggest problem for wheat producers in Kenya. Severe infestations by RWA may result in yield losses of up to 90% while “Ug99” infected fields may suffer 100% crop loss. The two pests combined are seriously affecting wheat farmers’ incomes because of the heavy reliance on pesticides that increase the cost of production. This study attempted to develop and characterize wheat lines that are resistant to both RWA and “Ug99” by pyramiding two major resistance genes. Three wheat varieties: “Kwale”, a Kenyan high yielding variety but susceptible to both RWA and “Ug99”;“Cook”, an Australian variety carrying stem rust resistance gene Sr36 conferring immunity to “Ug99”;and “KRWA9”, a Kenyan line with resistance to RWA but of poor agronomic attributes were used. A double cross F1 (DC F1) was obtained by crossing the F1 of “Kwale × Cook” and the F1 of “Kwale × KRWA9”. The DC F1 population was subjected to sequential screening for both RWA and “Ug99” resistance. Surviving DC F1 progenies were left to self pollinate to obtain the F2 of the double cross (DC F2). The DC F2 progenies were sequentially screened against RWA and “Ug99” to yield a population that was resistant to both RWA and “Ug99”. Genotyping of the DC F2:3 families were conducted to select homozygous resistant plants. Data indicated that the RWA and “Ug99” resistance genes were successfully pyramided. Though races with virulence for Sr36 have been reported, the gene provides immunity to race “Ug99” and can still be effectively used as a component for “Ug99” resistance breeding together with other Sr genes.

Evaluation of Dryland Wheat Cultivars on the Market in South Africa for Resistance against Four Known Russian Wheat Aphid, <i>Diuraphis noxia</i>, Biotypes in South Africa

An increased wheat yield potential under changing environmental conditions is a challenge in agriculture. Resistant wheat lines can yield more than susceptible wheat lines in the presence of Russian wheat aphid infestation. There are currently four Russian wheat aphid (RWA) biotypes known in South Africa with different virulence against different wheat cultivars. To keep up with the ever-changing patterns it is necessary to screen the cultivars for resistance against these Russian wheat aphid (RWA) biotypes. All the dryland wheat cultivars on the market were evaluated for resistance against the four known Russian wheat aphid (RWA) biotypes in South Africa. Through this evaluation, the status of Russian wheat aphid (RWA) resistance in South African dryland wheat cultivars can be updated to adapt to environmental changes and the wheat industry can adapt to changes in virulence of Russian wheat aphid (RWA) biotypes that may cause damage to Russian wheat aphid (RWA) resistant cultivars, subsequently affecting yield. Evaluations were done in the glasshouse by screening wheat cultivars against four different South African Russian wheat aphid (RWA) biotypes, RWASA1-RWASA4, under controlled conditions. The glasshouse evaluations showed that out of the 19 dryland wheat cultivars currently on the market in South Africa 16 are resistant against RWASA1, 7 are resistant against RWASA2, 7 are resistant against RWASA3 and 5 are resistant against RWASA4. Dryland wheat cultivars were also evaluated under field conditions at four different field localities. In the field, 5 cultivars were resistant to RWASA3 at two localities, respectively, and 3 and 5 cultivars were resistant to RWASA4 at two localities, respectively. Since Russian wheat aphid (RWA) damage can influence the final yield of a wheat cultivar significantly, changing conditions can influence both resistant cultivars, and the virulence of Russian wheat aphid (RWA). It is advisable to evaluate wheat cultivars on the market under different conditions and with all known Russian wheat aphid (RWA) biotypes in an area.

Field Screening of Lesotho and South African Wheat Cultivars for Russian Wheat Aphid Resistance

Russian wheat aphid (Diuraphis noxia) is an international wheat pest and was first recorded in South Africa in 1978 in the Bethlehem area in the Eastern Free State. Le-sotho lies adjacent to one of the largest wheat producing areas in South Africa, the Eastern Free State, where winter wheat and facultative types are cultivated under dry land conditions. Wheat (Triticum aestivum L.) is an important crop adapted to all agro-ecological zones of Lesotho. Russian wheat aphid may have a significant impact on wheat yield. No monitoring or pest control is being done in Lesotho and at this stage there is very little information on the Russian wheat aphid resistance of wheat culti-vars cultivated in Lesotho. In view of this it is important to monitor the distribution of Russian wheat aphid biotypes in Lesotho and determine the level of Russian wheat aphid resistance in local Lesotho wheat cultivars. Two local Lesotho wheat cultivars, Bolane and Makalaote were screened together with South African cultivars Elands, Matlabas, Senqu, PAN3379, PAN3118 and SST387, in the glasshouse against all four known biotypes that occur in South Africa. All these cultivars were also planted in 5 m plots in the field at two localities Leribe and Roma in the lowlands of Lesotho. These cultivars were screened in the field for Russian wheat aphid resistance. The predomi-nant Russian wheat aphid biotypes in these areas were also determined. The Lesotho cultivar, Bolane had resistance against RWASA2 in the glasshouse, while Makalaote did not have any Russian wheat aphid resistance in either the glasshouse or field screenings. To contribute to food security an increasing wheat yield potential is a high priority. Russian wheat aphid has been included in the list of important international cereal pests. Russian wheat aphid adapts to changing environments and taking their ecology, distribution, virulence patterns, and variability into account is important in minimizing the gap between actual and attainable yields. Current management prac-tices for winter wheat in South Africa include the use of resistant cultivars, which is the most economical management strategy for Russian wheat aphid. Introducing Russian wheat aphid resistant cultivars in Lesotho will improve overall yield and as a result food security. This will also result in lower Russian wheat aphid pest pressure in the adjacent wheat production areas in the Eastern Free State, South Africa.

Resistance in Barley (<i>Hordeum vulgare</i>L.) to New Invasive Aphid, Hedgehog Grain Aphid (<i>Sipha maydis</i>, Passerini) (Hemiptera: Aphididae)

Sipha maydis Passerini (Hemiptera: Aphididae) is a pest of cereals in many regions of the world and was identified as an invasive pest of the US in 2007. Regional surveys from 2015-2017 revealed this pest was broadly distributed throughout many of the western Great Plains states where it is a potential threat to cereal production. The common name hedgehog grain aphid, HGA, has been associated with Sipha maydis in the US. Cross-resistance where a plant is resistant to one aphid species and is also resistant to another species that is known to occur. Six barleys were evaluated for cross-resistance to HGA: Russian wheat aphid, RWA, resistant germplasms STARS 9301B and STARS 9577B and cultivar “Mesa”;greenbug, GB, resistant germplasm STARS 1501B and cultivar “Post 90”;and RWA and GB resistant experimental line 00BX 11-115. Cultivars “Morex” and “Schuyler” were susceptible controls. Antixenosis was measured 5 days after infestation by HGA. Seedling damage ratings and reductions in seedling growth were recorded after 17 days of infestation. Intrinsic rate of increase, rm, of HGA was determined by following the development of newborn aphids to adulthood and reproduction. 00BX 11-115 and Post 90 had significantly greater antixenosis (fewer aphids/seedling), significantly lower plant damage ratings, and significantly lower intrinsic rates of increase than other entries. Differences in seedling growth were not significant. 00BX 11-115 and Post 90 were the only entries with the Rsg1 greenbug resistance gene. Rsg1 greenbug resistance confers cross-resistance to HGA in the seedling stage.

Polymorphisms in salivary-gland transcripts of Russian wheat aphid biotypes 1 and 2

The Russian wheat aphid （RWA）, Diuraphis noxia （Mordvilko） （Homoptera： Aphididae）, is a major pest of small grains. As with plant-feeding aphids in general, the interaction between RWA and host plants is governed, on the insect side, by proteins and enzymes in saliva. In this work, we examined sequence variations in transcripts encoding proteins and enzymes of RWA salivary glands. We conducted reverse transcription - polymerase chain reaction in RWA biotypes 1 and 2 using primers derived from pea aphid orthologs, and cloned regions of 17 putative salivary gland transcripts. For four of the transcripts, we observed no difference in sequences between the two biotypes. For the other 13 transcripts, for example, the transcripts encoding sucrase, trehalase and protein C002, large amount of variations, both within each biotype and between the two biotypes, were observed. Usually the two biotypes shared only one variant, which was typically the most common variant in both biotypes. Most of the transcripts had more non-synonymous than synonymous codon changes among their variants. Our results offer possible molecular markers for distinguishing the two biotypes and insights into their evolution.

Differential colonization of wheat cultivars by two biotypes of Russian wheat aphid （Homoptera： Aphididae）

Susceptible and resistance wheat cultivars, Triticum aestivum L, were presented to two biotypes of Russian wheat aphid, Diuraphis noxia （Mordvilko）, in multiple choice tests to assay their relative acceptability as host plants. Both apterae （third and fourth instars） and alate adults were offered plants at the two-leaf stage in different cultivar combinations at 22±1℃ and 16：8 （L： D） hour photoperiod. Apterae were released from Petri dishes in the center of a circle of test plants, whereas alatae dispersed from a mature aphid colony to settle on plants arranged in rows. Both alatae and apterous nymphs of both biotypes readily colonized all cultivars tested：‘2137＇, ‘Akron＇,‘Ankor’,‘ Halt’ ,‘ Jagger’ ,‘ Prairie Red’ , ‘Stanton＇,‘TAM 107＇,‘TAM 110＇,‘Trego＇, ‘ Yuma＇, and ‘Yumar＇. Fewer biotype I apterae responded （settled and fed） in the combination containing more resistant （Dn4- and Dny-expressing） cultivars, compared to the combinations that had fewer. The reverse was true for biotype 2 apterae; more aphids responded in the combination containing the largest number of Dn4 expressing cultivars. Differential colonization of cultivars was observed in only one combination, in which biotype 2 apterae colonized Akron and Yumar in larger numbers than they did Stanton and Yuma. A separate experiment confirmed that, 48 hours after infestation, more biotype 2 apterae abandoned plants of Yuma than plants of Yumar. This differential response was likely due to genetic differences between the two ＇ near isogenic＇ lines that include the lack of Dn4 expression in Yuma. Choice tests with alatae did not result in differential rates of cultivar colonization by either biotype in any combination tested. These results suggest that young wheat plants appear to lack any meaningful antixenosis toward D. noxia, even though the aphids appear to perceive, and sometimes respond to, certain differences in cultivar suitability.

Induced life cycle transition from holocycly to anholocycly of the Russian wheat aphid (Homoptera: Aphididae)

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko),exists with holocyclic life cycle in Tacheng, Xinjiang in Northwest China. It produces males and oviparae to mate and oviposit for overwintering by eggs. Under laboratory conditions with 14 h/d photophase and temperature not lower than 15℃, RWA occurred in parthenogenesis and produced no males. The laboratory popu-lations of Russian wheat aphid, which were kept under natural conditions in fall by 15th, 49th and 81st generation while wild populations produced males and oviparae for mating, produced males and oviparae with their number decreased gradually, but viviparae and nymphs increased sequen-tially. As a result, it produced a small amount of oviparae and no males emerged in fields by 49 generations' reproduction in laboratory. After development of 81 generations, oviparae happened occasionally and no eggs occurred for overwintering instead of viviparae and nymphs. A hypothesis of RWA disastrous process was proposed. The life cycle of RWA can be changed from holocycly to anholocycly in its long-term spread and evolution. Anholocycly is more dangerous than holocycly to small grains for its strong adaptability and dispersal ability.

世界麦双尾蚜研究进展:分布、危害和生物学特性

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), has become a worldwide cereal pest with its dispersion to over 30 countries in this century. According to the natural history of its occurrence around the world, it is postulated RWA originated from western or central Asia. The aphid dispersed gradually to Europe and northern Africa, but its big jump across the ocean to North America is still a mystery. There are two overwintering strategies in RWA. The anholocyclic biotype, often reproducing earlier and more offsprings than the holocyclic one, has greater impact on crops in South Africa and United States. According to the experiments on its thermal response, RWA could withstand temperatures below -20℃, while temperatures above 30℃ would be harmful to its survival. The preference to colder condition may determine its distribution on the world. RWA has made great damages to cereal crops worldwide. It caused loss in crop yield by directly feeding on plant nutrition and disturbing the plant metabolism. But its status as plant virus transporter is open to question. By now, the aphid is still a serious pest in many countries and its invasion to more countries and areas maybe continues. Therefore, the research on its biological characteristics as well as its dispersion apparently needs to be enhanced in the future.

麦双尾蚜在中国的适生区预测

根据麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ） 在中国新疆的分布地点， 对ＣＬＩＭＥＸ 软件中适宜温度上限、限制性高温、有效积温、冷逆境开始积累点、热逆境开始积累点、冷逆境积累速率、热逆境积累速率和湿逆境积累速率等参数值进行修改调试， 调整后的ＣＬＩＭＥＸ 生态气候模型， 对新疆麦双尾蚜分布的模拟准确率达到９０ ％ 。由此模型进行预测， 云南、新疆、黑龙江、青海、西藏、吉林、辽宁、甘肃、宁夏、内蒙古、山西和山东等１２ 个省（ 区） 存在麦双尾蚜的适生区。以麦双尾蚜在新疆发生程度和小麦分布区的关系， 对ＣＬＩＭＥＸ的预测结果进行修正， 据此推测山东不适合麦双尾蚜生存， 东北、西北和西南春麦区麦双尾蚜生存适宜程度比ＣＬＩＭＥＸ 模型预测值降低１／２ ～３／４。

麦双尾蚜发生程度与气象因素的关系

利用新疆塔城１９８９ ～１９９６ 年８ 年的麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ） 发生程度与１６个气象因子进行相关性分析， 通过逐步回归筛选因子， 确定麦双尾蚜发生量预测模型：ｌｏｇ Ｙ＝８－４１００－ ０－１０３３ＲＨ５ － ０－０２５３Ｒ５ ， 其中： Ｙ为麦双尾蚜发生百株蚜量； ＲＨ５ 为５ 月份的相对湿度（ ％ ）； Ｒ５ 为５ 月份的降水量（ｍ ｍ）。应用该模型预测１９９７ ～１９９８ 年麦双尾蚜的发生程度， 与实际发生情况基本吻合。

麦双尾蚜及其天敌在春小麦和野生寄主上的种群动态

在塔城麦田调查发现， 麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ） 在春小麦和野生寄主上的百株蚜量从大到小依次为春小麦、黑麦和野燕麦， 在春小麦上最早出现高峰期。在春小麦上， 瓢虫类数量最多达到５１６ 头／ 百株， 菜蚜茧蜂Ｄｉａｅｒｅｔｉｅｌｌａ ｒａｐａｅ 和白足蚜小蜂Ａｐｈｅｌｉｎｕｓ ａｌｂｉｐｏｄｕｓ 的最高寄生率分别达到３１－６ ％ 和１２－９％ 。在黑麦上， 麦双尾蚜主要被白足蚜小蜂寄生， 最高寄生率达到４２－９ ％ ， 斑腹蝇幼虫Ｌｅｕｃｏｐｉｓａｎｎｕｌｉｐｅｓ 最高达到１６ 头／ 百株。在野燕麦上， 白足蚜小蜂和菜蚜茧蜂最高寄生率分别达到６２－３ ％ 和３１－８ ％ 。瓢虫和菜蚜茧蜂发生较早， 白足蚜小蜂和斑腹蝇发生时间相对较晚。

麦双尾蚜的龄期鉴别

麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ） 无翅孤雌蚜的体长、体宽、头宽、腹管宽、上尾片长、尾片长和触角节Ⅲ长等７ 个形态指标， 在各龄期间差异显著。根据麦双尾蚜的翅、翅芽、触角节数、尾片形状、上尾片长与宽比值等特征作为龄期鉴别的主要指标， 编制了麦双尾蚜龄期鉴定检索表。

温度对麦双尾蚜发育、存活和繁殖的影响

实验室内测定麦双尾蚜Ｄｉｕｒａｐｈｉｓｎｏｘｉａ（ Ｍｏｒｄｖｉｌｋｏ） 在９ 个恒温下取食小麦叶片时的生长发育数据。麦双尾蚜发育起点温度为３－２７ ℃， 发育适温区为１５ ～２０ ℃， 有效积温为１５２－５５ 日·度。在较低温度下（７－５ ℃、１０ ℃、１５ ℃） 各发育阶段的总存活率较高， 说明低温对麦双尾蚜生长有利， 但１ ～２ 龄若蚜的存活率略低于３ ～４ 龄， 说明低龄若蚜的抗逆性稍差。在１５ ～２４ ℃下麦双尾蚜单雌产仔量高， 为繁殖最适温区。

麦双尾蚜形态结构的扫描电镜观察

在扫描电镜下， 系统观察了麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （Ｍｏｒｄｖｉｌｋｏ） 的触角初生感觉器、触角次生感觉器、复眼、喙端感觉器、前跗节端部感觉器以及腹部末端的外部形态超微结构， 并就其形态和功能的关系进行了探讨。

麦双尾蚜与麦田中生物和非生物因子的灰色关联分析

利用灰色系统理论， 将麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ） 种群消长动态与麦田生物群落中主要生物因子和非生物因子进行灰色关联分析。结果表明影响麦双尾蚜及其它麦蚜种群消长的关键因子主要为麦蚜间的竞争和天敌的捕食压力。非生物因子的作用较小， 其中较重要的因子有平均相对湿度和降水量。在塔城小麦田中关键天敌类群为瓢虫类， 寄生天敌的影响较小； 在伊犁小麦田中， 麦双尾蚜及其它麦蚜的关键天敌类群为蚜小蜂类， 大麦田中斑腹蝇类较重要， 燕麦田中瓢虫类占优势。

四种天敌对麦双尾蚜的功能反应

一种蚜小蜂Ａｐｈｅｌｉｎｕｓｓｐ ． 对麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （Ｍｏｒｄｖｉｌｋｏ） 的功能反应为ＨｏｌｌｉｎｇＩ型， 直线方程为Ｎａ ＝ ０－６０６０Ｎ－ ３－４７００ 。七星瓢虫Ｃｏｃｃｉｎｅｌｌａ ｓｅｐｔｅｍｐｕｎｃｔａｔａ Ｌ． 成虫对麦双尾蚜功能反应也为Ｉ型， 直线方程为Ｎａ ＝ ０－６０２０Ｎ＋ ５－９０００ ； 多异瓢虫Ｈｉｐｐｏｄａｍｉａ ｖａｒｉｅｇａｔａ Ｇｏｅｚｅ成虫和斑腹蝇Ｌｅｕｃｏｐｉｓａｎｎｕｌｉｐｅｓ Ｚｅｔｔ．３ 龄幼虫对麦双尾蚜的功能反应均为ＩＩ型， 关系式分别为１／ Ｎａ ＝ １－２５５０／ Ｎ＋ ０－００４６ 和１／ Ｎａ ＝ １－３２８０／ Ｎ＋ ０－００７１。

麦双尾蚜自然种群的特定时间生命表

根据新疆伊犁、塔城两地小麦田调查资料， 组建了麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ）自然种群特定时间生命表。由生命表分析得知， 小麦早期生长发育阶段， 麦双尾蚜迁移造成的种群数量损失很小， 反映了麦双尾蚜在早期种群密度较低， 个体对食物和空间资源的竞争压力较小。寄生性天敌在小麦田出现较晚， 在两地小麦早期生长阶段， 由寄生造成的种群损失也极低。整个麦类作物生长阶段， 捕食天敌对麦双尾蚜种群的控制作用非常重要， 是麦双尾蚜种群损失的关键因子之一。迁移造成的种群损失比寄生性天敌的还要大， 说明迁移是麦双尾蚜种群自我调节的重要手段。

麦类品种对麦双尾蚜的耐害性及产量损失率

在新疆伊犁就３６ 个麦类品种对麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ） 耐害性和产量损失的研究结果表明， 其中１１ 个品种受害后千粒重下降１０ ％ 以下， 为耐害性较强的品种， 它们是一粒小麦、墨玉稻穗、爱因亢、小偃９５ 、Ｍｇ８３４９ 、短芒黑边红、伊春４ 号、小黑麦１２ 、浮纳尔、毛大头和Ｔ１００８。综合考虑不同品种的自然受害率和受害后千粒重的下降， 各品种的总体产量损失率在０ ～１０－５６ ％ 。产量损失率在２ ％ 以下的有１４ 个品种， 即一粒小麦、爱因亢、Ｍｇ８３４９ 、Ｍｇ８７８６ 、浮纳尔、短芒黑边红、Ｔ１００８ 、墨玉稻穗、小黑麦１２ 、广引７４ 、Ｍｇ４５２１ 、黄库尔班、Ｍｇ８８１６ 和小偃９５ 。

麦双尾蚜发生数量与小麦播种期的关系

新疆伊犁和塔城冬、春麦田的麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ） 有虫株率和百株蚜量，与小麦播种时间密切相关。在小麦正常播种期内， 冬小麦晚播可以显著减少麦双尾蚜数量， 每晚播种１０ 天， 第二年麦双尾蚜有虫株率可以下降４０ ％ ～７０％ ； 春小麦每晚播种１０ 天， 麦双尾蚜有虫株率增加３０ ％ ～８８ ％ 。

麦双尾蚜及其天敌在不同海拔高度的分布

在新疆塔城， 麦双尾蚜Ｄｉｕｒａｐｈｉｓ ｎｏｘｉａ （ Ｍｏｒｄｖｉｌｋｏ） 在春麦田最集中分布于海拔６００ ～８００ ｍ 高度， 在冬麦田最集中分布于７００ ～８００ ｍ 高度； 冬麦田和同海拔高度的春麦田麦双尾蚜数量密切相关（ｒ＝ ０－９１ ， Ｐ＜ ０－０１） 。麦双尾蚜寄生性天敌蚜茧蜂类和蚜小蜂类， 在春麦田最集中分布的高度为５００ ～６００ ｍ ， 随着时间的延后， 分布的高度范围逐渐扩展； 冬麦田最集中分布的高度为６００ ～８００ ｍ ， 略高于春小麦田。捕食麦双尾蚜的斑腹蝇幼虫在春麦田前期最集中分布的高度为５００ ｍ 处， 后期分布高度上升； 冬麦田较集中分布的高度为６００ ～８００ ｍ ， 也随着时间的延后也上升。