Tag: whitefly vector of viruses

 

Tomato diseases are many, and are important because tomatoes are so popular! A couple of weeks ago, we started to notice yellowing of tomato leaves and upward/inward leaf-rolling in the middle of one of the two rows in our hoophouse, on the Tropic variety. The veins of the yellowing leaves remain green, a pattern called chlorosis. Yellowing indicates chlorophyll is not functioning, photosynthesis is not happening much and so no new leaves will grow, no sugars will be made and fruits will not be sweet and tasty.

One of our crew did some research and reported back that chlorosis can be caused by several physical problems, some nutrient deficiencies, or by disease, including a few viral diseases. The chlorosis is spreading to other varieties

The University of Illinois Urbana-Champaign Extension provides helpful starter info.

Seminis has a thorough 78-page 1997 well-organized downloadable pdf Tomato Disease Guide, with good photos.

Another source including good photos is the 168-page 2017 De Ruiter and Seminis Tomato Disease Field Guide available online from Bayer or Noresco. I’m not advocating using anything but organic methods, but these companies have good ID information.

Penn State has Scouting and Identifying Tomato Diseases, a short video, useful to learn how to scout well and what to look for.

Root-knot nematode (M. enterolobii) symptoms on ‘Rutgers’ tomato roots. Other species of root-knot nematode produce similar galling symptoms
T. Schwarz, NC State University

 Here’s our summary:

Chlorosis Cause 1: physical or chemical problems

1. It could be a high pH problem. I think this is unlikely, but we can do a soil test in October.
2. Injured roots? Check for root knot nematodes when we pull up the plants in a couple of weeks.
3. Compacted roots? Very unlikely, unless someone has been stomping all round the stems.
4. Poor drainage? Unlikely, unless

   

   there were a big drip tape leak, which there has not been.

5. Salt buildup? We have had very hot weather, maybe salts have built up faster than usual. We will soon do our twice annual salt washdown. This does seem possible.
6. Clemson Extension provides info on leaf roll, see HGIC 2222, Tomato Leaves Rolling? “Excessive moisture and nitrogen, insufficient phosphorus, heat, drought, severe pruning, root damage from tilling or hoeing, early planting and transplant shock are some of the cultural and environmental factors that can cause physiological leaf roll in tomatoes. Initial symptoms are usually apparent in the lower leaves with an upward cupping of leaflets followed by an inward lengthwise rolling of the leaflets toward the mid-vein. The affected leaves tend to become thickened and have a leathery texture, but retain a normal, healthy green color. Over time, all of the leaves on the plant may be affected. . . The good news is that the condition has minimal impact on tomato fruit production and plant growth.” Viral infections and herbicide injury can also cause leaf roll (more on viral infections below).

Chlorosis Cause 2: Nutrient deficiency

1. Check which foliage turned chlorotic first. Younger tomato leaves are generally towards the top of the plant (or on sideshoots). Older leaves are lower. Mobile nutrients (nitrogen, phosphorus, potassium, magnesium, chlorine) show symptoms first on older leaves, and immobile nutrients (iron, manganese) show symptoms first on younger leaves.
2. Of the immobile nutrients, iron chlorosis starts at the leaf base and the leaf becomes very pale; manganese deficiency looks like magnesium deficiency, but starts in the younger leaves, not the older ones as it does with magnesium deficiency.
3. From the video Iron Deficiency in Tomatoes, from Matthew DeBacco, I really don’t think we have an iron deficiency. The younger leaves are not the main site of the problem, and our plants don’t have the elongated look that the ones in the video have.

   

4. From the nutrient deficiency photos on page 74 of the 1997 78-page downloadable pdf Seminis Tomato Disease Guide, ours look closest to the magnesium-deficient ones, IMO. Green veins with interveinal yellowing.
5. The Illinois Extension says “Other elements such as calcium, zinc, manganese, phosphorus, or copper in high amounts in the soil can tie up iron so that it is unavailable to the plant. However, a shortage of potassium in the plant will reduce the availability of iron to the plant.” A soil test will show if we have shortages or excess of anything. Then we can add any needed amendments for the winter and next summer’s crops.
6. Nutrient deficiencies would generally affect plants along the whole bed, but we have healthy Green Zebras right next to the Tropics.

   

7. Last winter’s crops in that bed were chard, senposai and turnips, all of which could have taken a lot out of the soil. But the bed was composted adequately on 10/9/23 with 5 barrows of compost for a 90’ x 4’ bed.
8. Was there anything unusual about the planting of the tomatoes? Not enough compost in the holes? Not enough or too much firming of the soil around the tomatoes? Do we remember?

Chlorosis Cause 3: Tomato Chlorosis Virus ToCV or Tomato Infectious Chlorosis Virus (TICV)

1. ToCV and TICV are both Crinivirus diseases in the family Closteroviridae. TICV was first detected in Orange County, California in 1993; ToCV in Florida in 1998. The viruses have since spread worldwide. Both viruses can seriously reduce tomato yields. They can be mistaken for magnesium or nitrogen deficiency, physiological disorders or pesticide damage, which appear very similar. They require the whitefly vector. They are not seed-borne. Although tomato is the major economic host for both these viruses, they can infect a range of other plants including some common weeds.
2. See page 55 of the Seminis Tomato Disease Guide for an image of Infectious Chlorosis Virus (TICV). It does look like what we have. Vectored by whitefly only. Symptoms start on older leaves as interveinal yellow blotches, while veins remain green. The interveinal yellow patches can become white or die and be brittle. Symptoms can easily be mistaken for problems caused by poor growing conditions, aging or nutrient deficiency.

   

3. But these diseases are spread by whiteflies. We have had no sign of whiteflies this year or ever, in our hoophouse. Are we sure we don’t have whitefly? They are tiny (1 mm). Shake the lower leaves early in the morning and a cloud of whiteflies will rise, if they are there. We don’t see any.
4. Dead aphid skins fall off some plants when they are shaken. Can aphids spread the virus? We did have a lot of aphids this spring. No, just whitefly.
5. As ToCV is also known to affect southern peas, eggplants, lettuce, potatoes, pumpkin and sweet peppers, should we avoid those crops in that bed this winter and next summer? This list includes other nightshades (unsurprisingly), but also lettuce, beans and squash, so maybe also other members of those families, which covers almost everything, but notably not brassicas!! I hope we are growing brassicas in that bed this winter. Keep an eye on the peppers!
6. Watching the How we can identify ToCV (Tomato Chlorosis Virus video from Mohammad in South Africa, ToCV seems less likely. We do not seem to have the main vector (whitefly), although the speaker suggests mechanical vectors are also possible for ToCV. Our plants look different. It is true that with the virus the chlorosis starts in the middle of the plant, as ours did. In the video, the affected leaves are distorted, buckled, and turned yellow all over, including the veins, even when the plant didn’t yet have much of an infection. As the disease spreads fast, the leaves look golden all over in just 2 weeks. The tops of the plants yellow and wilt. That’s not how ours are.

Chlorosis Cause 4: Other viruses?

1. Tomato Yellow Leaf Curl Virus? It’s shown on the Clemson Extension document, HGIC 2222, Tomato Leaves Rolling?. Plants are very stunted. Spread by whitefly, not seed-borne. Can spread from tobacco and Jimson weed.
2. Beet curly-top virus? The photo on page 51 of the Seminis Tomato Disease Guide looks more curly, stunted and less chlorotic than our plants, but not impossible. Spread by leaf-hoppers

There is no cure for tomato virus infections. Careful removal and destruction of the plants is recommended. Control of insect vectors and weed hosts is also important.

Chlorosis Cause 5: Other causes?

1. Texas A&M has a site on Disorders of Tomato Leaves. Nothing there looks like what we have. It did cause me to wonder about high salts.
2. Missouri Botanical Gardens has A Visual Guide: Tomato Foliage, Stem & Root Problems. Not many virus pictures, though.
3. Fusarium Rot fungus (pages 52-55) causes interveinal chlorosis, spreading up the plant from the lower leaves, and reddish-brown lesions in the vascular material of the main root and lowest part (only) of the stem, which would be evident when we pull up the plants. Not the same disease as Fusarium Wilt. Can be spread in soil, water, and through the air.
4. Psyllid Yellows and Vein-Greening are bacterial diseases spread by tomato psyllids. Doesn’t much look like what we have, I think. Leaves curl outwards. Ours are curled in. Pictures in the 168-page Tomato Disease Field Guide

My current top four suspects:

A.    Magnesium deficiency

B.    TICV and we didn’t see the whiteflies

C.     High salt build-up in the soil

D.    Nematodes