BỆNH ĐỐM LÁ NHỎ – SOUTHERN LEAF BLIGHT (LEAF SPOT) – Thuốc trừ sâu sinh học,thuốc trừ nấm,thuốc trừ rầy,thuốc diệt chuột

BỆNH ĐỐM LÁ NHỎ – SOUTHERN LEAF BLIGHT (LEAF SPOT)

BỆNH ĐỐM LÁ NHỎ trên cây ngô
Bệnh Đốm Lá Nhỏ - Southern Leaf Blight

BỆNH ĐỐM LÁ NHỎ – SOUTHERN LEAF BLIGHT (LEAF SPOT)

Posted On May 8, 2017 at 7:35 am by / 913 Comments

Tác nhân gây bệnh:

Bệnh do nấm Helminthosporium maydis Nishikado & Miyake, giai đoạn hoàn toàn là Cochliobolus heterostrophus Drechsler. Loài nầy tỏ ra thích ứng với điều kiện nóng ẩm cao hơn loài H. turcicum.

Triệu chứng:

Trên lá, đốm bệnh có nhiều dạng và màu sắc khác nhau: có đốm hình chữ nhật, hình thoi hoặc hình ellip, màu vàng, nâu vàng hoặc nâu đỏ, có viền nâu tím bao quanh, dài 5-10 mm và được giới hạn bởi hai gân phụ của lá. Sự thay đổi hình dạng và màu sắc của đốm bệnh là do giai đoạn phát triển của bệnh, điều kiện thời  tiết, phản ứng của giống bắp trồng,v.v…; ngoài ra, còn do đặc tính gây hại của dòng nấm bệnh:
– Trên lá: dòng nấm O tạo ra những đốm bệnh hình chữ nhật với viền màu nâu, có kích thước nhỏ 0,6 x 1,2-1,9 cm; còn dòng nấm T thì tạo vết bệnh to hơn, hình chữ nhật hoặc hình thoi với viền  màu màu nâu đỏ.
– Trên thân: dòng T tạo vết bệnh giống như trên lá, còn dòng O  không tạo vết bệnh trên thân.
– Trên trái: dòng T tạo ra lớp mốc như nỉ đen, còn dòng O không tạo vết bệnh trên trái.
Bệnh làm chết các mô chứa diệp lục tố, làm giảm khả năng quang hợp, làm thân cây yếu ớt, lá không còn bổ dưỡng trong chăn nuôi, giảm năng suất hạt. Khi hạt giống bị nhiểm bệnh, cây con có thể sẽ chết. Bệnh rất phổ biến ở những vùng có khí hậu ấm áp, ẩm ướt, như ở vùng nhiệt đới. Bệnh có thể tấn công từ khi cây mới có  2-3 lá đến lúc thu hoạch. Cây thiếu dinh dưỡng, bệnh càng trầm trọng thêm.

Biện pháp phòng trừ:

– Dùng giống kháng bệnh. Giống kháng bệnh có nhiều dạng: dạng kháng bệnh bằng phản ứng cho đốm bệnh màu vàng, do di thể rhm; dạng kháng bệnh bằng phản ứng cho đốm bệnh nhỏ, vùng bị hại ít, dạng nầy được chi phối bởi nhiều di thể. Tính kháng dòng O được  chi phối bởi nhân, còn tính kháng dòng T được chi phối bởi nhân  và tế bào chất. Ở ĐBSCL, các giống ít nhiểm bệnh được ghi nhận  là: Western yellow, Thái sớm hổn hợp, Mehico 4 và Mehico 7. Các giống dễ nhiểm bệnh là: Taiwan 11, Đỏ Đài Loan, Răng ngựa,…
– Khử hạt với maneb, captan, organomercury, Rovrral (2g/ 10kg hạt) hoặc với hỗn hợp carboxin và thiram. Bón phân đầy đủ và cân đối N-P-K.
– Phun thuốc ngừa trị bệnh như ở bệnh Đốm lá to. Trong điều kiện nhà lưới, có thể dùng vi khuẩn đối kháng để phòng trị bệnh.

Biện pháp hóa học:

+ Phun thuốc khi bệnh mới xuất hiện.

Catcat 250 EC 15 ml/16 lít nước
Carbenda Supper 50 SC 20-25 ml/16 lít nước
Manozeb 80 WP, Ridozeb 72WP 60-80 g/16 lít nước
Aviso 350SC 14 ml/16 lit nước

+ Khi bệnh đã phát triển mạnh, có thể sử dụng thuốc có một số hoạt chất sau để trị bệnh: Propiconazole , Difenoconazole, Hexaconazole, …

Ngoại cảnh:

Việc xác định dòng nấm gây bệnh được dựa vào  triệu chứng bệnh xuất hiện  trên cây con (4 tuần tuổi) khi được chủng bệnh. Ngay sau khi chủng bệnh, cây con được giữ nơi có ẩm độ cao (95%) trong 24 giờ để bệnh phát triển.

Chu trình:

Có hai dòng gây hại đã được xác định là dòng T và dòng O. Dòng C (tấn công giống bắp có tế bào chất C) là dòng thứ ba, mới được xác định tại Trung Quốc.  Đính bào tử có hình thoi dài, hơi cong, màu nâu vàng, gồm nhiều tế bào,có 2-15 vách ngăn, kích thước: 25-140 x 10-21 micron.

Đính bào đài có màu nâu, mọc riêng lẻ hay kết thành chùm, gồm nhiều tế bào với 4-17 vách ngăn, kích thước: 162-487 x 5-9 micron, mang 1-8 đính bào tử.

Giả bao nang (pseudothecia) có miệng, hình cầu, màu đen, kích thước: 0,4-0,6 x 0,4 mm, chứa nhiều nang (asci). Mỗi nang có 4 nang bào tử; nang bào tử gồm 6-10 tế bào, kích thước: 6-7 x 130-340 micron. Giai đoạn sinh sản hữu tính hiếm khi xảy ra trong  điều kiện tự nhiên.

Mầm bệnh tạo bào tử từ cây bệnh hoặc xác cây bệnh. Bào tử  được gió mang đi lây nhiểm vào các lá bắp, đốm bệnh xuất hiện và 5-6 ngày sau đó sẽ cho ra bào tử. Dòng O ít gây hại hơn dòng T. Ở lô hạt được thu thập từ ruộng nhiểm bệnh, có đến 99% hạt có sự  hiện diện của dòng T, trong khi không thấy dòng O mặc dù nó cũng có khả năng gây hại trên hạt. Bệnh cũng được truyền từ hạt; tuy nhiên, điều nầy chỉ xảy ra ở dòng T, còn ở dòng O thì chưa có bằng chứng rõ ràng. Cây con phát triển từ hạt bị nhiểm dòng T, sẽ bị héo chết trong vòng 3-4 tuần sau khi trồng.

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