Management of Pests and Diseases

Late blight is a very destructive and very infectious disease that affects tomato and potato (not sweet potato). It is the same disease that led to the Irish Potato Famine in the 1840s. It is caused by the fungal-like pathogen, Phytophthora infestans. It is prevalent during the rainy season and when there is excess moisture or humidity.  

What are its symptoms? 

Lesions develop on leaves and stems as dark, water-soaked spots. These spots enlarge until the entire leaf or stem turns brown and dies. Dead leaves typically remain attached to stems. The undersides of the lesions may be covered with a white fuzzy growth that contains the spores of the pathogen. On  the stems, late blight lesions appear brown to almost black. Infected tomato fruits develop shiny, dark or olive-colored lesions which may cover large areas and in particular the upper half of the  fruit. 

How is it different from other diseases with almost similar symptoms? 

When diagnosing for late blight examine all parts of the affected plants thoroughly. The late blight  pathogen produces most of its spores at night and as a result it is more visible in the morning and these call for scouting early in the day for the disease. Generally, late blight can affect all parts of the plant whereas some of the “imitators” cannot. Below are some of the diseases that portray symptoms most similar to LB and how they differ from the latter: 

1. Gray mold - This is the disease most commonly confused for late blight because the pathogen causes large leaf spots, stem lesions, and affects fruits. These symptoms are often associated with dead plant tissue (flowers, leaves). The pathogen typically needs to become established on these dead tissues before it can attack living plant tissue. Affected fruit are soft and are not brown. The pathogen growth is fuzzier and gray to brownish, not white as with late blight. 

2. Early Blight - Leaf, stem and fruit spots are all smaller than those for late blight and often have a  characteristic concentric ring pattern or target-shape appearance.

3. Septoria Leaf Spot - Leaf and stem spots are all much smaller in comparison to those for late blight and often have a characteristic tan center. Fruits are not affected.

4. Buckeye Fruit Rot - Fruit turns brown with white spores forming when moist. Unlike late blight, buckeye fruit rot is most likely on fruit on or near the soil where the pathogen can survive between crops, the fruit stays firm and smooth (not rough) and leaves and stems are not affected. Causal pathogens are closely related to late blight, but do not travel far or fast because their spores move by splashing water and soil, rather than air.

5. Drought Stress - When plants’ roots cannot deliver enough water to leaves, large sections of leaf  tissue can die. In contrast with late blight lesions, symptoms of drought stress always extend from the leaf edge, they lack a border of wilted tissue, and there is no fuzzy pathogen growth. Also, no symptoms develop on stems or fruit. 

What can you do about it? 

• Start with disease-free tomato seedlings. 

• Scout daily in the morning hours for any symptoms on the crop. 

• Rotate tomato fields with non-solanaceous crops. Crop rotation is for the early blight and Septoria  leaf spot diseases which are an annual problem, not late blight. The late blight organism requires living tissue to survive; it does not survive in the soil or carry in tomato seed. 

• Control tomato volunteer plants as well as solanaceous weeds such as hairy nightshade. 

• When late blight is found in small, localized areas, promptly destroy all symptomatic plants plus a border of healthy appearing plants to prevent disease spread. 

• Apply late blight specific fungicides in affected fields and nearby fields on a regular basis until tomato harvest is complete. Shorten spray interval when disease pressure is high or environmental conditions remain favourable for the late blight pathogen (cool and wet). 

• Alternate fungicide applications among different chemical classes; include a contact (protectant) fungicide in each application (chlorothalonil, mancozeb, or copper). Addition of a protectant fungicide enhances resistance management and fungicide effectiveness; for example, copper oxychloride,  Mancozeb+cymoxanil or propineb+cymoxanil or Metalaxyl+Mancozeb.

• Good fungicide coverage is necessary. 

• Work in affected field last and clean equipment between fields. 

• Disk under the field or kill with herbicide after harvest is complete. 

Tomato yellow leaf curl 

Tomato yellow leaf curl virus (TYLCV) is a viral disease transmitted by whiteflies; it is one of the most damaging diseases in tomatoes. TYLCV is the most significant tomato virus in areas where whitefly is a big problem. The disease incidence is higher particularly when temperatures are high. Locally, it is known as: Gathuri or ngumi .

Symptoms 

• Plants are severely stunted with shoots becoming erect.

Leaf symptoms include: 

• Small leaves which are cupped, thick and rubbery. Tops of infected plants may look like a head of broccoli. 

• Leaflets are reduced in size and puckered.

• Leaflets that develop soon after infection curl downwards at the margins; leaves produced later curl upwards, become distorted, and have prominent yellowing along margins and/or intervenal regions.

• Flowers wither.

• Plants will set very few fruit after infection occurs; when older tomato plants are infected, they produce abnormal growth above the point of infection. Any fruit already on the plant ripens almost normally, but flowers often drop or fail to set fruit. The appearance of the fruit is unaffected. 

Conditions for Disease Development 

• The virus is not seed-borne. It is only transmitted by the whitefly, Bemisia tabaci.

• The whitefly vector has a very wide host range but it usually does not cause symptoms in these hosts. Most solanaceous plants such as tomato, eggplant, potato, tobacco, and pepper can be infected with TYLCV but remain healthy in appearance. Common bean is also a host and will sometimes display leaf curl symptoms when infected. Many common weeds are also host to the virus and may or may not develop symptoms when infected. 

• Hot and dry conditions favor the whitefly, and therefore aid the spread of TYLCV. Whitefly  populations decrease after heavy rain showers.

Management 

1. Use resistant varieties: Hybrid Tomato Assila from Monsanto is resistant to Tomato Yellow Leaf Curl Virus (TYLCV).

2. Under stress, however, these resistant varieties can lose their protection and develop symptoms of tomato yellow leaf curl. As such, it is important to keep plants healthy by proper irrigation and fertilizer regimes and to keep them free of other pests and diseases. 

3. Grow seedlings in an insect-proof net house or in a greenhouse, and maintain good control of  whiteflies in these structures in order to prevent early infection of seedlings by whitefly feeding. If non insect-proof nets are used in seedling nurseries, then they should be sprayed with insecticides to control entry of whitefly into the structures. 

4. The whitefly vector favors younger plants. To reduce this effect, tomato plants should be about 30 days old at the time of transplanting. 

5. Timing of transplanting can also be effective in avoiding high populations of whitefly and therefore reducing or high populations of whitefly and therefore reducing or preventing TYLCV  infection. Separate plantings of tomatoes in time and space from plantings of crop hosts which are good sources of whiteflies (i.e. cabbage, cucurbits, potato).

6. Avoid overlapping tomato crops that allow the vector to subsist and develop new populations. Pulling out volunteer tomato and tobacco plants and weed control are also important in reducing  sources of virus inoculums. 

7. Plant new tomato crops in isolated fields. If possible, plant a tall border crop, such as maize, around the tomato crop. 

8. Chemical control methods include the application of systemic insecticides as soil drenches or  regular sprays during the seedling stage to reduce the population of the whitefly vector. A second application may be necessary to control adults that have emerged from the egg and nymph stage  since the application of the first spray. Rotation of insecticides may be necessary to prevent the development of resistance in the vector. However, chemical control may not be effective in areas  where disease incidence is high.

9. Symptomatic plants should immediately be carefully removed, bagged, and discarded to prevent the spread of whiteflies on them that may be carrying the virus. Cover plant in plastic bag and tie at  the stem at soil line. Cut off the plant below the bag and allow bag with plant and whiteflies to  desiccate to death on the soil surface for 1–2 days prior to placing the plant in the trash. Do not cut the plant off or pull it out of the garden and toss it on the compost! The goal is to remove the plan reservoir of virus from the garden and to trap the existing virus-bearing whiteflies so they do not disperse onto other tomatoes. 

Buckeye Fruit & Root Rot 

Symptoms on Fruit: 

Brown spots appear on green and ripe fruit, often at the blossom end. The spots have bands of dark  and light brown rings. A white cottony fungal growth appears under moist conditions. Young green fruit, when infected, usually become mummified. Fruit touching or near the soil are most likely to become infected.

On Root: 

Phytophthora can cause a root and crown rot of tomato plants at all ages. Damping-off symptoms occur on seedlings while infections of the roots and crowns of young plants cause rapid wilt. On established plants, brown water-soaked lesions appear on roots, extending into the lower part of the stem. Severely affected roots become necrotic and decayed. The leaves become bronze and later dieback from the tip. 

On Stem:

The canker that develops is pale green to brown and may extend more than 15cm. The canker girdles the stem and causes wilting and death.

Doses: 

1. Thiophanate methyl 

2. Carbendazim  

Alternaria Stem Canker 

It appears on stems, leaves, and fruit. Dark brown to black cankers with concentric zonation occur on stems near the soil line or above ground. Canker enlarge girdle the stem, and kills the plant. Dark brown to black areas of dead tissues between leaf veins are caused by a toxin produced by the fungus. Dark brown sunken lesions with characteristic concentric rings develop on green fruit.

Doses: 

1. Copper fungicides 

2. Carbendazim 

Gray Mold

It appears on stems, leaves, and fruit. Dark brown to black cankers with concentric zonation occur on stems near the soil line or above ground. Canker enlarge girdle the stem, and kills the plant. Dark brown to black areas of dead tissues between leaf veins are caused  by a toxin produced by the fungus. Dark brown sunken lesions with characteristic concentric rings develop on green fruit. Foliage of plants from seedlings to mature plant stage is affected. Brown to black specks appear on both the young and older leaves. The lesions expand slowly into 1-mm to 2-mm-diameter round spots that remain brown or develop a gray center surrounded by a yellow area.    

Early Blight (Alternaria Solani) 

In tomatoes, it causes stem cankers on seedlings and small irregular dark brown spots on the older leaves leading in partial defoliation of the crop. The fungus survives on the crop debris. Infections begin as small brown spots on older leaves that quickly enlarge. The lesions develop a "bulls-eye" pattern of concentric rings that can be seen.

Fusarium & Verticillum Wilt

Both of these fungal diseases are soil-born. Fusarium often causes yellowing on one side of the plant or leaf. Yellowing begins with the older, bottom leaves, followed by wilting, browning, and defoliation. Growth is typically stunted and little or no fruit develops. Brown, vascular tissue can be found when the infected stem is cut at its base. Infected plants often die before maturing. 

Verticillium is less common, usually occurring late in the season when soils are cooler. It begins as a v-shaped blotch on lower leaves, followed by browning veins and dead, chocolate brown blotches. It spreads up the plant. Discoloration of the vascular tissue is limited to the bottom 12 inches of the stem. Look for tomatoes with the resistance letters VF on the plant tag or seed packet, indicating resistance to both verticillium and fusarium wilt. 

Lookalikes: All the leaf spot diseases and “bottom-up” diseases 

Bacterial Cankers (Clavibacter Michiganensis) 

The symptoms of bacterial canker are brown leaf margins with a yellow border next to the inner  green leaf tissue, spreading between the veins (top image). It often affects only one side of a plant. As plants wilt, yellowing leaves usually remain attached to the plant. Unlike the leaves, the petioles remain green. The vascular system shows a yellowish brown discoloration (bottom image). This is a serious infectious disease with no cure and no resistant varieties available. It will kill the plant and is  highly infectious, easily spreading to other plants. Infected plants should be removed from the garden immediately and extensive measures implemented for disinfection. 

Lookalikes: Other foliar diseases, particularly fusarium wilt, viruses 

Bacterial Wilt (Pseudomonas Solanacearum)

The disease causes wilting of tomatoes and potatoes. In tomatoes it is mainly seed borne. 

Control: Control is mainly cultural as strict crop rotation, removal and burning infected plant debris, and planting certified seed. 

Damping Off  

Affected plants usually occur in patches in nursery beds or in low parts of sloped fields. In pre-emergence damping off, the seeds fail to emerge after sowing. They become soft, mushy, turn brown, and decompose as a consequence of seed infection. In post-emergence damping-off, the seedling emerges from the soil but dies shortly afterward.    

Septoria Leaf Spots

Septoria leaf spot usually appears on the lower leaves after the first fruit sets. Spots are circular, about one-sixteenth to one-fourth inch in diameter with dark brown margins and tan to gray centers with small black fruiting structures. Characteristically, there are many spots per leaf; they do not look target-like. This disease spreads upwards from oldest to youngest growth. If leaf lesions are numerous, the leaves turn slightly yellow, then brown, and then wither. Fruit infection is rare.  

Lookalikes: Bacterial leaf spot and speck (no tan centers); and other diseases that progress from the bottom up.

Powdery Mildew

Leaf symptoms consist of yellow blotches or spots on the upper leaf surface. A white powdery growth occurs on the underside of the leaves. This fungus penetrates within the leaf tissue. The older colonies of the fungus may turn a dirty white color with age. The severely affected leaves turn yellow, then brown, and later become shriveled. Generally, the lower leaves are affected first and the disease gradually moves up the plant. Vines may become defoliated under severe infections, leading  to lower yields and possible sunburn damage to the fruit.

Bacterial Spot and Speck 

These are two separate but very similar diseases. Symptoms of bacterial spot are small dark brown to  black circular spots, initially water-soaked, coalescing and becoming angular, sometimes with a yellow halo. 

Symptoms of bacterial speck are tiny, dark brown to black spots with a surrounding yellow halo. Severity of both diseases is increased by wetness of fruit and foliage from sprinklers, rain, or heavy dew. There are some tomato varieties with resistance to bacterial speck but very few with resistance to bacterial spot. 

Lookalikes: Septoria leaf spot (tan centers).