Wet Conditions: Common Problems for Medicinal and Culinary Herbs

 

With the wet conditions over the past year, many diseases are showing up on crops that have not had any major disease problems in the past. For many low-acreage crops such as medicinal and culinary herbs there is minimal information on the diseases and disorders that can affect the crop. Furthermore, growers have few options for chemical controls. However, there are cultural strategies that can be employed to minimize the impact of these diseases.

Leaf Diseases

Leaf Blights and Spots

Extended periods of leaf wetness can lead to several different types of leaf blights and spots. Leaf diseases are caused by a number of different fungi and bacteria. Proper identification often requires the use of a microscope and trained diagnosticians. However, the cultural controls to minimize these diseases are common to most crops. Many leaf diseases are caused by a handful of different fungi. Alternaria leaf blight is common on many crops and it is usually caused by a different species of Alternaria for each crop. Examples of known Alternaria diseases of medicinal and culinary herbs include Alternaria leaf blight of basil (Figure 1), Alternaria leaf blight of ginseng, and Alternaria leaf blight of lemon balm. The most common feature of lesions of Alternaria diseases is a concentric ring pattern and a yellow halo (Figure 2). However, symptoms can vary greatly from one crop to another. Other common leaf blight and spot fungi include Cercospora, Septoria (Figure 3), Colletotrichum (Anthracnose) and Botrytis. Cercospora leaf blights usually exhibit small, round lesions with distinct borders, while Septoria leaf blight lesions are variable in shape but often contain small black fruiting structures called pycnidia. Leaf lesions can also be caused by bacterial blight (Figure 4).

Figure 1. Alternaria leaf blight lesions on basil.

Figure 2. Alternaria leaf blight lesion on a ginseng leaf. Note the appearance of a ring pattern and a yellow halo around the lesion.

Figure 3. Septoria leaf spot on curly-leaf parsley. Close examination of the spots reveals tiny black dots which are the pycnidia or spore-producing structures of the fungus.

Figure 4. Bacterial blight on cilantro. Symptoms of bacterial blight can look very similar to those of Cercospora leaf spots.

Powdery Mildews

Powdery mildews are among the most easily recognized diseases of plants. Although they can be caused by many different species of fungi, the diseases all appear very similar. At first the lesions are small and yellow, but the entire leaf usually rapidly develops a frosted appearance (Figure 5), and the leaf turns yellow and eventually dies. Spores of the causal fungi spread through the field and from field to field mainly by wind but can also spread on machinery and field workers.

Figure 5. Powdery mildew on sage. Despite different species of the fungus on different crops, powdery mildew symptoms are very similar for all crops.

Other Foliar Diseases

Downy mildews are among the most rapidly spreading and destructive diseases. Although they do occur, they are not common among medicinal and culinary herbs in Ontario. Phytophthora leaf blights are also very destructive and occur under very wet conditions. These diseases are very crop specific and are more often associated with root rots.

Root Rots

Wet conditions are also conducive to the development of root diseases. Roots can die as a result of sitting in saturated soil for an extended period of time. However, wet soils can also promote soil fungi that can cause portions of the root or the entire root system to die. Common symptoms of root rot include yellowing of the leaves and wilt of a portion of a plant or the entire plant (Figure 6). Root rots are often caused by several different fungi including Pythium, which often causes damping off of seedlings, Fusarium, Phytophthora, and Rhizoctonia. Unlike foliar diseases, it can be very difficult to identify the cause of root rots based on symptoms alone.

Figure 6. Wilted plants of cilantro as the result of a root disease. Often entire plants will wilt, especially during the heat of the day when the roots cannot supply enough water to the tops. These symptoms are typical of Pythium or Rhizoctonia.

Other Problems in Wet Conditions:

White Mold

White mold is caused by the fungus Sclerotinia sclerotiorum. The disease usually begins as water soaked patches on fleshy parts of the plant such as young stems and dense patches of leaves (Figure 7). A white fluffy growth often accompanies the water soaked lesions. On older lesions, black, hard structures called sclerotia will develop on the white fluffy growth. The disease is usually promoted by humid or wet conditions in protected areas of the plant. A dense canopy or plant is usually required for the disease to develop unless weather conditions are very wet for an extended period of time.

Figure 7. Sclerotinia white mold on basil. These plants were attacked along the stem resulting in eventual collapse of the entire plant. A white mold was seen on portions of the stem.

Bacterial Soft Rots

Bacterial diseases are very common in wet conditions. Soft rots develop on fleshy plant material, mainly in areas with dead tissue, areas that were wounded, or in protected portions of the plant that remain very wet. Most slimy rots that develop on plants are caused by bacteria.

Slugs

Slugs can multiply rapidly in wet conditions. Symptoms of slug damage are holes in the leaves without the yellow are brown borders characteristic of foliar diseases (Figure 8). If slugs are the cause of the damage they can often be found hiding in protected and moist areas around the soil surface during the day and they mainly come out at night to feed.

Figure 8. Slug damage on ginseng.

Regardless of the cause of the disease there are several strategies a grower can use to keep damage to a minimum.

1. Plant less susceptible varieties. There is often great variation among varieties in the susceptibility to certain diseases. By choosing varieties that have more resistance to a disease in the first place, you avoid many problems throughout the life of the crop. In some cases, information on the susceptibility of a variety or cultivar to certain diseases can be supplied by the seed company. If this information is not available, small on-farm trials can be conducted to determine the best variety for your area. For some crops, different varieties may not be available, but because of genetic variability some plants in a field may be more resistant to a disease. If you need to collect seeds or cuttings, focus on those plants.
   
2. Reduce leaf wetness. Most of the fungi that cause foliar diseases can only infect the plant if the spores land on the plant and there is moisture on the surface of the leaf for an extended period of time. The best strategy to reduce foliar disease is to reduce leaf wetness. Slow airflow through the canopy results in higher humidity around the plant and longer leaf wetness periods after a rainfall or dew period. Consequently, by increasing airflow disease can be reduced. This can be accomplished by avoiding close plant and row spacing, planting in more exposed locations with a steady airflow, and potentially pruning plants once the canopy closes, if possible. A less dense canopy can also increase the drying of the soil surface and reduce rots that develop on the stem, crown or roots of the plant. A lower plant stand will also reduce the spread of soil fungi from plant to plant and will reduce slug problems.
   
3. Sanitation. Most of the foliar diseases produce spores that can be spread by wind and/or picked up on clothing and machinery and moved throughout the field. Root diseases will produce spores or other structures that can picked up on boots and farm machinery and moved down the row. If a disease is present in your field, care should be taken to minimize movement of machinery or field workers through the field, especially when the leaves or soil are wet. It is also advisable to clean and sanitize machinery and boots when moving from an area with disease to a field or area of a field where the disease is not present. If the disease is localized to a few plants, these can be removed from the field and destroyed to reduce the amount of spores released to the rest of the field. It is also important to incorporate crop residues soon after harvest to ensure that there is no living plant tissue on which the fungus can survive the winter.
   
4. Proper Fertility. Proper fertility will help the crop defend against a disease. Under-fertilization can weaken the plant, resulting in the inability of the plant to form defences that can prevent an organism from infecting the plant. On the other hand, over-fertilization can result in succulent growth which can make it easier for an organism to infect the plant. In addition, over-fertilization can result in excessive leaf growth which will lead to higher humidity in the canopy and increased disease development.
   
5. Drainage. Most of the root rot fungi are promoted by wet soils. In addition, the higher humidity in the canopy caused by wet soils will also promote foliar diseases. Consequently, removing water quickly from the field will reduce many root rot problems. Tile drainage is the best method of removing water from the soil. Growers should ensure that there is proper drainage in the field prior to planting a field to a susceptible crop. Examine the fields after a heavy rain to see where water pools and where it runs off the field. Improving drainage in these areas will help to reduce root rots from developing and spreading to other areas of the field. Subsoiling can also be used to break up any plow pan and allow water to drain more rapidly.
   
6. Irrigation management. Proper timing of irrigation is essential for minimizing foliar diseases. If at all possible, avoid overhead irrigation at times when it is likely to prolong the leaf wetness period. Watering early in the morning when the leaves are already wet from dew will minimize the length of the wetness period. Drip or trickle irrigation is ideal because soil moisture can be maintained without any additional leaf wetness.