Tomato Yellow Leaf Curl
Pathogen: Tomato yellow leaf curl virus (TYLCV)
SYMPTOMS AND SIGNS
Tomato yellow leaf curl is a disease of tomato caused by Tomato yellow leaf curl virus. In March 2007, it was identified for the first time in California but does not yet appear to have become established in the state. An educational brochure (1.1 MB PDF) was created at that time and is available to print.
Infected tomato plants initially show stunted and erect or upright plant growth; plants infected at an early stage of growth will show severe stunting. However, the most diagnostic symptoms are those in leaves.
Leaves of infected plants are small and curl upward; and show strong crumpling and interveinal and marginal yellowing. The internodes of infected plants become shortened and, together with the stunted growth, plants often take on a bushy appearance, which is sometimes referred to as "bonsai" or broccoli'-like growth. Flowers formed on infected plants commonly do not develop and fall off (abscise). Fruit production is dramatically reduced, particularly when plants are infected at an early age, and it is not uncommon for losses of 100% to be experienced in fields with heavily infected plants.
COMMENTS ON THE DISEASE
Tomato yellow leaf curl virus is undoubtedly one of the most damaging pathogens of tomato, and it limits production of tomato in many tropical and subtropical areas of the world. It is also a problem in many countries that have a Mediterranean climate such as California. Thus, the introduction of the virus into California must be considered as a serious potential threat to the tomato industry.
There are a number of factors, however, that may not favor establishment of the virus in the major tomato-producing areas of California, including the Sacramento and San Joaquin valleys. First, its vector, Bemisia whitefly species are not typically found in these tomato-producing areas because it is intolerant of winter temperatures there. Second, the Central Valley's winter season provides a "natural" tomato-free period, which usually goes from late November through early February. Although the virus can infect other plants, tomato is the host in which it builds-up most quickly. Thus, by having an annual "tomato-free period", it is likely that the amount of viral inoculum (as well as whitefly populations) will be significantly reduced by the time the tomato planting season start again in late winter-early spring. This would mean that, even if the virus is able to overwinter, it may take a long time to reach levels that cause economic damage.
Tomato yellow leaf curl virus is a geminivirus (family Geminiviridae). Although it can infect a relatively wide range of plant species, tomato is the host to which the virus is best adapted and that facilitates the build-up of the virus to high incidences in the field. Other hosts includes solanaceous crops, which may develop symptomless infections, and weeds (e.g., nightshade and jimsonweed).
In addition, the virus causes leaf curl in certain varieties of common bean (Phaseolus vulgaris) and the ornamental plant lisianthus (Eustoma grandiflorum). A range of weeds from other families can be infected by this virus, but most of these do not develop obvious disease symptoms. It is not known how well whiteflies acquire the virus from symptomless hosts. However, it has been hypothesized that these hosts serve as a "bridge" for the virus in the absence of tomato crops, and that perennial weeds help allow the virus to become permanently established.
The primary way the virus is spread short distances is by Bemisia whitefly species. Over long distance, the virus is primarily spread through the movement of infected plants, especially tomato transplants. Because it can take up to 3 weeks for disease symptoms to develop, infected symptomless plants could be unknowingly transported. The virus also can be moved long distance by virus-carrying whiteflies that are transported on tomatoes or other plants (e.g., ornamentals) or via high winds, hurricanes, or tropical storms.
COMMENTS ON CONTROL
Rapid and precise tests for Tomato yellow leaf curl virus are available at UC Davis and CDFA. These tests can be carried out in less than 24 hours. Anyone finding tomatoes with TYLC-like symptoms can contact their county farm advisor, Robert L. Gilbertson at UC Davis (telephone; 530-752-3163) or Tongyan Tian at CDFA (telephone: 916-262-1127).
بیماری ویروسی پیچیدگی وزردی برگ گوجه فرنگی
Tomato yallow leaf curlvirus
علایم بیماری:
بیماری پیچیدگی وزردی برگ گوجه فرنگی یکی از مخرب ترین بیماری های گوجه فرنگی
می باشد . این بیماری باعث خسارت سخت و سنگین بر گوجه فرنگی های
کنسروی و تازه خوری می شود . گیاهان آلوده کوتاه باقی می مانند در حالی که بخش
های هوایی و برگ های آن کوچک هستند و حالت راست و ایستاده به خود می گیرند.
برخی برگ ها به سمت بالا وداخل لوله می شوندو بدشکل می گردند. و شدیدا دچار
کلروز می گردند. پس از آلودگی گیاهان، گلها به طور چشمگیری ریزش پیدا می کنند ،
میوه ها نارس مانده ومیوه بازار پسندی تولید نمی کنند. آلودگی های اولیه تقریبا همیشه
منجر به خسارت صد درصد می شوند.
عامل بیماری:
عامل این بیماری TYLCV میباشد. این ویروس از خانواده Geminiviridae است که توسط مگس
سفید منتقل می شود و شامل یک ژنوم حلقوی تک رشته ای ( (ssRNA می باشد در حالیکه
ژنوم سایر جنس های این خانواده که با مگس سفید منتقل می شوند از دو رشته ssRNA تشکیل
شده است. TYLCV توسط مگس سفید Bemisia tabaci به صورت ناپایا و غیر تکثیری منتقل می شود .
ویروس می تواند در عرض 15-30 دقیقه توسط ناقل از گیاه آلوده دریافت شده و پس از15-30 دقیقه تغذیه
از گیاه سالم آن را منتقل میکند و بعد از 21 ساعت کمون علایم را پدید می آورد . مگس سفید ناقل
ویروس می توانند تا دو هفته ویروس را منتقل کنند. این ویروس چندین گیاه دیگر را نیز آلوده می کند
از جمله توتون و علف های هرزی مانند تاتوره و گیاهان مربوط به 5 خانواده از گیاهان دیگر. برخی از این
گیاهان به عنوان میزبان تناوبی و نیز محل زمستانگذرانی ویروس در طول زمستان به شمار می روند که
این مساله کنترل بیماری را مشکل تر می کند.
کنترل بیماری :
کنترل این بیماری خیلی مشکل است .تقریبا همه واریته های تجاری حساسند اما تعداد کمی
از ارقام وحشی درجاتی از مقاومت را به این ویروس دارند. همچنین کنترل مگس سفید دارای
اثر جزیی بوده و هزینه زیادی را می طلبد. جدا سازی گوجه فرنگی های تازه کشت شده از
کشت های قدیمی ( از نظر زمان و مکان) و انجام عمل کاشت موقعی که جمعیت مگس های
سفید در حداقل ممکن است از روشهایی است که بالاترین میزبان کنترل را فراهم میکند.