Judicious succession: control of phytonematodes through crop succession

In Brazil, the main phytonematodes of cotton are the root-knot nematode Meloidogyne incognita, the reniform nematode Rotylenchulus reniformis and the lesion nematode Pratylenchus brachyurus. The three nematodes are obligate plant parasites, that is, they feed exclusively on living plant tissue. Therefore, controlling these organisms can be done by eliminating their food, which is normally achieved in two different ways. The first, and simplest, is to remove all plants from an infested area. It is a control technique called fallowing.

The other, which is more complicated, and the reason we will see below, is the selection of plants that will be kept in the area, preceding or following cotton cultivation. They should preferably be plants from which nematodes are not able to obtain food and, consequently, where they do not reproduce (= non-host or immune plants). Alternatively, plants in which reproduction is low (= resistant plants) can be used. The complication of this technique, which is called “phytonematode control through crop succession” is that the three important species are polyphagous, that is, they feed on several plant species. So, the list of plants that can be used in succession is very small.

The advantages and disadvantages, from a nematological point of view, of the main successions involving the cotton plant are discussed below.

Soy-cotton succession

 Currently, in Brazil, it is one of the most important successions. Nematodes M. unknown, R. reniformis e P. brachyurus reproduce abundantly in soybeans, that is, soybeans are a good host for (= susceptible to) these nematodes. Therefore, this succession should not be considered, in principle, for the control of cotton nematodes. However, through genetic improvement, today there are soybean cultivars resistant to M. unknown e R. reniformis, which can be used to control these nematode species. For example, cultivars BRS-282, BRS-256RR, BRS-260 are resistant to M. unknown; and the cultivars BRSMG-250 (Nobreza), TMG-115RR, TMG-121RR, TMG-113RR, BRS-Jiripoca are resistant to the reniform nematode. Unfortunately, the reaction of soybean cultivars, especially the more modern ones, is not always available to the public.

Cotton millet

 It is a succession widely used for straw formation, especially in regions with dry winters. Millet is immune to R. reniformis, and is susceptible to M. unknown e P. brachyurus. Therefore, millet is valuable in controlling reniform nematodes, but represents a risk in places infested with root-knot and lesion nematodes. There are some cultivars resistant to P. brachyurus, which reduce the risk of its use in places infested by this nematode.

Cotton corn

 It is normally used in irrigated areas. Corn is immune to R. reniformis, but is susceptible to M. unknown e P. brachyurus, that is, the reaction of corn is similar to that of millet, except for the fact that there are no corn cultivars or hybrids resistant to the lesion nematode.

Cotton Peanut

 Peanuts are one of the best crops for controlling nematodes. In the United States, it has been used successfully to control M. unknown (it is immune to this species of root-knot nematode), but can also be recommended in places infested by the reniform nematode (it is resistant to R. reniformis). Its weak point is its susceptibility to P. brachyurus, a nematode that causes lesions on the roots, gynophore and pod skin. In Brazil, its use in controlling cotton nematodes is infrequent.

Soybean-castor-cotton, soybean-sunflower-cotton and soybean-sorghum-cotton

 Castor beans, sunflowers and sorghum can be used as second crops after soybeans. These are sequences in which there is high complexity, as they involve three different cultures. Firstly, it is necessary to consider the effect of soy, which can increase or decrease the risk caused by nematodes.

The literature records that castor bean is immune to M. unknown and susceptible to R. reniformis e P. brachyurus. In a soybean-castor-cotton sequence, castor bean reduces the risks caused by M. unknown, but increases the risks due to R. reniformis e P. brachyurus.

Sunflower is susceptible to three species of nematodes, therefore it is a viable option only in places with low infestations of these organisms. Otherwise, it is a culture to be avoided.

Grain sorghum is immune to R. reniformis, and susceptible to M. unknown e P. brachyurus. Galls caused by M. unknown they are very small and can lead the farmer to the false impression that sorghum is resistant to the nematode.

Brachiaria soybean-cotton and soybean-Panicum maximum-cotton

 These are complex sequences, like those described in the previous item, however, using brachiariae (Brachiaria decumbens, B. ruziziensis e B. brizantha) to Panicum maximum (Massai, Tanzania, Aruana, etc.) as crops for straw formation. In addition to the effect of soy, there is the action of these coverings, which are immune to root-knot and reniform nematodes, but susceptible to lesion nematodes.

Soy-Crotalaria spectabilis-cotton

 It is the sequence with the greatest range of controlled species, as the literature records that C. spectabilis is resistant to M. unknown, R. reniformis e P. brachyurus, although there is controversy about the reaction of this green manure to the reniform nematode.

Importance of crop succession

 Traditionally, in Brazil, control of cotton nematodes has been carried out through the use of resistant and/or tolerant cultivars. However, in more recent times, resistance to nematodes has not been considered a priority attribute in cotton genetic improvement programs. In fact, current cultivars have lower resistance than cultivars used 5 years ago. Thus, the relative importance of other control methods has grown, namely chemical nematicides (in the planting furrow or in seed treatments), nematicides biological (in the furrow or in seed treatment), organic fertilization and crop succession.

Crop succession, as a nematode control method, has two main advantages: (1) the information necessary for its implementation is available in the literature; (2) the results are very predictable. However, it is very little used, for the following reasons: (1) high opportunity cost; (2) scarcity of phytonematology laboratories (which are necessary for the specific identification of nematodes); (3) side effects associated with succession culture, as it may be immune or resistant to a nematode, but susceptible to another nematode existing at the site.

Below is an example of the application of succession to control nematodes on a large cotton-producing property. In this property, in 2003, the corn-cotton succession was used throughout its entire extension (1978 ha divided into 24 plots), which caused, as expected, an increase in the population of M. unknown e P. brachyurus. In 2004, as a control attempt, the peanut-cotton succession was used in plots 7 and 12, which were two of the least productive (21st and 22nd, respectively). From a nematological point of view, the new succession had the advantage of replacing a plant susceptible to M. unknown (corn) for an immune (peanut). The risk posed by P. brachyurus, as peanuts are also susceptible to this species of lesion nematode. As a result, despite the injuries to the pod shells, peanut production was high and provided a net income greater than that of the corn itself. The most important fact is that the density of M. unknown was reduced, despite poor tiguera control, and cotton production benefited greatly. In 2004, production in plots 7 and 12 increased by 17,1% compared to 2003, while in the other plots, production fell by 13,3%. In addition, plot 7, which was the 21st. most productive in 2003, it moved to 18th. place in 2004. The result for plot 12 was even better, moving from position 22 to 11 (Table 1).

Although, in this example, the results with peanuts were encouraging, the experiment was discontinued, due to a decision based on the commercial aspect and related to the greater price fluctuation of peanuts, compared to corn. In fact, in Brazil, unlike the United States, peanuts are currently a crop of marginal importance. Thus, it is confirmed that opportunity cost has been the main obstacle to the adoption of succession in the control of cotton nematodes in Brazil.

 

Table 1. Production in 24 cotton production plots in the municipality of Serra do Ramalho (BA). In 22 plots, the succession was corn-cotton in 2003 and 2004; in 2 plots, the succession was corn-cotton in 2003 and peanut-cotton in 2004.

Field	Production @/ha
2003 Corn-Cotton	2004 Corn-Cotton
5	329	224
6	284	245
8	292	246
9	210	181
10	182	217
11	253	206
13	240	265
14	225	168
15	276	250
16	172	176
17	305	185
18	259	200
19	253	260
20	357	270
21	350	280
22	319	281
23	289	257
24	301	317
25	257	232
27	245	231
28	308	250
29	363	323
Media	275,9	239,3
Difference 2004-2003	-13,3%
	Corn-Cotton	Peanut-Cotton
7	206	216
12	191	249
Media	198,5	232,5
Difference 2004-2003	+ 17,1 %

Field

Production @/ha

2003

Corn-Cotton

2004

Corn-Cotton

5

329

224

6

284

245

8

292

246

9

210

181

10

182

217

11

253

206

13

240

265

14

225

168

15

276

250

16

172

176

17

305

185

18

259

200

19

253

260

20

357

270

21

350

280

22

319

281

23

289

257

24

301

317

25

257

232

27

245

231

28

308

250

29

363

323

Media

275,9

239,3

Difference 2004-2003

-13,3%

 

 

 

 

Corn-Cotton

Peanut-Cotton

7

206

216

12

191

249

Media

198,5

232,5

Difference 2004-2003

+ 17,1 %

 The full article is in issue 192 of Cultivar Grandes Culturas.