The Effects Of New Alfalfa Disease Caused By Leptosphaerulina Australis On Yield And Nutrient

Alfalfa production in China is not enough to Chinese stock farming,not only in total yield,but also in quality.Disease is one of limiting factors to alfalfa production since effect yield and quality,and furthermore to alfalfa industry.Therefore,it is necessary to determine disease kinds and know their infection cycle in order to control alfalfa diseases and reduce loss.The main results obtained after disease field investigation,pathogen identification,and evaluation for the effects in yield and quality of a new alfalfa disease in this thesis,as follows:1 The symptoms of this disease include two types,one is leaf blotch,and another is round spot.For the first type,lesions initially appeared on both sides of leaves,white,round,or sub rotund,spherical.The lesions late enlarged,with hygrophanous and yellow haloes around.Lesions are sunken,white,with a brown and circle edges.Finally leaves blighted after lesions merged together.For the second type,lesions are white,up to 5 mm,with brown edges,but did not merged together.Black particles exhibit on lesions in both two type lesions.2 Disease incidence in May 2019 and 2020 occurred on all cultivars including WL525,WL168,Qing Tian Zhu etc.,which lowest was 12.13%,and highest was52.54%.3 A fungus was isolated from diseased leaves with isolation percentage was51.00%to 85.71%.However,there was no conidia produced on various culture conditions,including on 9 media（PDA,PSA,CMA,etc.）,on 5 carbohydrate resources（glucose,sucrose,lactose,etc.）,and 5 nitrogen resources.Whatever,conidia produced on colonies kept under 4℃after 3 months,which morphological characteristics are same to conidia on diseased leaves.It was identified as Leptosphaerulina australis.4 Pathogenicity tests confirmed it is a pathogenic fungus to alfalfa.The disease named as alfalfa Leptosphaerulina leaf blotch since alfalfa is the new host of the fungus,and symptoms are different from other diseases.5 The effects of the disease to alfalfa yield:plant weight of disease severity of 2,3,and 4 reduced 1.33%,3.81%,16.71%and 29.60%,compared with healthy plants.Yield increased by 4.26%,9.18%,1.06%,0.93%,11.30%and 13.03%resulted from fungicide treated with mancozed,pyraclostrobine,boscalid,carbendazim,chlorothalonil and trifloxystrobin prayed twice on Liangmu cultivar.Control efficiency was 50.88%,41.77%,29.27%,31.16%,56.29%and 62.40%.Residual amounts of mancozed,pyraclostrobine and boscalid was 0.014 mg·Kg^-1,0.26 mg·Kg^-1and 0.014 mg·Kg^-1,others did not detected.6 The effects of the disease to alfalfa nutrients.Contents of crude protein,crude fat,calcium,phosphorus,potassium in whole plants are significant decreased（P<0.05）,while,contents of acid detergent fibre and neutral detergent fiber decreased significantly（P<0.05）.Crude protein and crude fat in plants with 4 grade severity decreased 16.72%and 18.78%.In conclusion,it is worth to pay much attention and properly manage this disease since it is new in China and probably spread to other provinces although it now only occurred in one region.